On Tue, Apr 24, 2018 at 09:16:38AM +0800, Tiwei Bie wrote:> On Tue, Apr 24, 2018 at 04:05:07AM +0300, Michael S. Tsirkin wrote: > > On Tue, Apr 24, 2018 at 08:54:52AM +0800, Jason Wang wrote: > > > > > > > > > On 2018?04?23? 17:29, Tiwei Bie wrote: > > > > On Mon, Apr 23, 2018 at 01:42:14PM +0800, Jason Wang wrote: > > > > > On 2018?04?01? 22:12, Tiwei Bie wrote: > > > > > > Hello everyone, > > > > > > > > > > > > This RFC implements packed ring support for virtio driver. > > > > > > > > > > > > The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented > > > > > > by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html > > > > > > Minor changes are needed for the vhost code, e.g. to kick the guest. > > > > > > > > > > > > TODO: > > > > > > - Refinements and bug fixes; > > > > > > - Split into small patches; > > > > > > - Test indirect descriptor support; > > > > > > - Test/fix event suppression support; > > > > > > - Test devices other than net; > > > > > > > > > > > > RFC v1 -> RFC v2: > > > > > > - Add indirect descriptor support - compile test only; > > > > > > - Add event suppression supprt - compile test only; > > > > > > - Move vring_packed_init() out of uapi (Jason, MST); > > > > > > - Merge two loops into one in virtqueue_add_packed() (Jason); > > > > > > - Split vring_unmap_one() for packed ring and split ring (Jason); > > > > > > - Avoid using '%' operator (Jason); > > > > > > - Rename free_head -> next_avail_idx (Jason); > > > > > > - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason); > > > > > > - Some other refinements and bug fixes; > > > > > > > > > > > > Thanks! > > > > > > > > > > > > Signed-off-by: Tiwei Bie <tiwei.bie at intel.com> > > > > > > --- > > > > > > drivers/virtio/virtio_ring.c | 1094 +++++++++++++++++++++++++++++------- > > > > > > include/linux/virtio_ring.h | 8 +- > > > > > > include/uapi/linux/virtio_config.h | 12 +- > > > > > > include/uapi/linux/virtio_ring.h | 61 ++ > > > > > > 4 files changed, 980 insertions(+), 195 deletions(-) > > > > > > > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > > > > index 71458f493cf8..0515dca34d77 100644 > > > > > > --- a/drivers/virtio/virtio_ring.c > > > > > > +++ b/drivers/virtio/virtio_ring.c > > > > > > @@ -58,14 +58,15 @@ > > > > > [...] > > > > > > > > > > > + > > > > > > + if (vq->indirect) { > > > > > > + u32 len; > > > > > > + > > > > > > + desc = vq->desc_state[head].indir_desc; > > > > > > + /* Free the indirect table, if any, now that it's unmapped. */ > > > > > > + if (!desc) > > > > > > + goto out; > > > > > > + > > > > > > + len = virtio32_to_cpu(vq->vq.vdev, > > > > > > + vq->vring_packed.desc[head].len); > > > > > > + > > > > > > + BUG_ON(!(vq->vring_packed.desc[head].flags & > > > > > > + cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT))); > > > > > It looks to me spec does not force to keep VRING_DESC_F_INDIRECT here. So we > > > > > can safely remove this BUG_ON() here. > > > > > > > > > > > + BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc)); > > > > > Len could be ignored for used descriptor according to the spec, so we need > > > > > remove this BUG_ON() too. > > > > Yeah, you're right! The BUG_ON() isn't right. I'll remove it. > > > > And I think something related to this in the spec isn't very > > > > clear currently. > > > > > > > > In the spec, there are below words: > > > > > > > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L272 > > > > """ > > > > In descriptors with VIRTQ_DESC_F_INDIRECT set VIRTQ_DESC_F_WRITE > > > > is reserved and is ignored by the device. > > > > """ > > > > > > > > So when device writes back an used descriptor in this case, > > > > device may not set the VIRTQ_DESC_F_WRITE flag as the flag > > > > is reserved and should be ignored. > > > > > > > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L170 > > > > """ > > > > Element Length is reserved for used descriptors without the > > > > VIRTQ_DESC_F_WRITE flag, and is ignored by drivers. > > > > """ > > > > > > > > And this is the way how driver ignores the `len` in an used > > > > descriptor. > > > > > > > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L241 > > > > """ > > > > To increase ring capacity the driver can store a (read-only > > > > by the device) table of indirect descriptors anywhere in memory, > > > > and insert a descriptor in the main virtqueue (with \field{Flags} > > > > bit VIRTQ_DESC_F_INDIRECT on) that refers to a buffer element > > > > containing this indirect descriptor table; > > > > """ > > > > > > > > So the indirect descriptors in the table are read-only by > > > > the device. And the only descriptor which is writeable by > > > > the device is the descriptor in the main virtqueue (with > > > > Flags bit VIRTQ_DESC_F_INDIRECT on). So if we ignore the > > > > `len` in this descriptor, we won't be able to get the > > > > length of the data written by the device. > > > > > > > > So I think the `len` in this descriptor will carry the > > > > length of the data written by the device (if the buffers > > > > are writable to the device) even if the VIRTQ_DESC_F_WRITE > > > > isn't set by the device. How do you think? > > > > > > Yes I think so. But we'd better need clarification from Michael. > > > > I think if you use a descriptor, and you want to supply len > > to guest, you set VIRTQ_DESC_F_WRITE in the used descriptor. > > Spec also says you must not set VIRTQ_DESC_F_INDIRECT then. > > If that's a problem we could look at relaxing that last requirement - > > does driver want INDIRECT in used descriptor to match > > the value in the avail descriptor for some reason? > > For indirect, driver needs some way to get the length > of the data written by the driver. And the descriptors > in the indirect table is read-only, so the only place > device could put this value is the descriptor with the > VIRTQ_DESC_F_INDIRECT flag set.when writing out used descriptor, device should set VIRTQ_DESC_F_WRITE (and clear VIRTQ_DESC_F_INDIRECT).> > > > > > > > > > > > > > > The reason is we don't touch descriptor ring in the case of split, so > > > > > BUG_ON()s may help there. > > > > > > > > > > > + > > > > > > + for (j = 0; j < len / sizeof(struct vring_packed_desc); j++) > > > > > > + vring_unmap_one_packed(vq, &desc[j]); > > > > > > + > > > > > > + kfree(desc); > > > > > > + vq->desc_state[head].indir_desc = NULL; > > > > > > + } else if (ctx) { > > > > > > + *ctx = vq->desc_state[head].indir_desc; > > > > > > + } > > > > > > + > > > > > > +out: > > > > > > + return vq->desc_state[head].num; > > > > > > +} > > > > > > + > > > > > > +static inline bool more_used_split(const struct vring_virtqueue *vq) > > > > > > { > > > > > > return vq->last_used_idx != virtio16_to_cpu(vq->vq.vdev, vq->vring.used->idx); > > > > > > } > > > > > > +static inline bool more_used_packed(const struct vring_virtqueue *vq) > > > > > > +{ > > > > > > + u16 last_used, flags; > > > > > > + bool avail, used; > > > > > > + > > > > > > + if (vq->vq.num_free == vq->vring_packed.num) > > > > > > + return false; > > > > > > + > > > > > > + last_used = vq->last_used_idx; > > > > > > + flags = virtio16_to_cpu(vq->vq.vdev, > > > > > > + vq->vring_packed.desc[last_used].flags); > > > > > > + avail = flags & VRING_DESC_F_AVAIL(1); > > > > > > + used = flags & VRING_DESC_F_USED(1); > > > > > > + > > > > > > + return avail == used; > > > > > > +} > > > > > This looks interesting, spec said: > > > > > > > > > > " > > > > > Thus VIRTQ_DESC_F_AVAIL and VIRTQ_DESC_F_USED bits are different for an > > > > > available descriptor and > > > > > equal for a used descriptor. > > > > > Note that this observation is mostly useful for sanity-checking as these are > > > > > necessary but not sufficient > > > > > conditions - for example, all descriptors are zero-initialized. To detect > > > > > used and available descriptors it is > > > > > possible for drivers and devices to keep track of the last observed value of > > > > > VIRTQ_DESC_F_USED/VIRTQ_- > > > > > DESC_F_AVAIL. Other techniques to detect > > > > > VIRTQ_DESC_F_AVAIL/VIRTQ_DESC_F_USED bit changes > > > > > might also be possible. > > > > > " > > > > > > > > > > So it looks to me it was not sufficient, looking at the example codes in > > > > > spec, do we need to track last seen used_wrap_counter here? > > > > I don't think we have to track used_wrap_counter in > > > > driver. There was a discussion on this: > > > > > > > > https://lists.oasis-open.org/archives/virtio-dev/201802/msg00177.html > > > > > > > > And after that, below sentence was added (it's also > > > > in the above words you quoted): > > > > > > > > """ > > > > Other techniques to detect > > > > VIRTQ_DESC_F_AVAIL/VIRTQ_DESC_F_USED bit changes > > > > might also be possible. > > > > """ > > > > > > > > Best regards, > > > > Tiwei Bie > > > > > > I see, the extra condition "if (vq->vq.num_free == vq->vring_packed.num)" > > > help in this case. > > > > > > Thanks > > > > I still think tracking a wrap counter is better. > > >From my understanding, wrap counter is only needed when > one side just want to update parts of the status bit(s), > it's something like the "report status" or "write back" > feature [1] in the hardware NIC. And in the driver, all > the status must be updated, and that's why I don't want > to track the usedwrap counter. > > [1] https://github.com/btw616/dpdk-virtio1.1/commit/ca837865bd10 > > Best regards, > Tiwei Bie > > > > > > > > > > > > Thanks
On Tue, Apr 24, 2018 at 04:29:51AM +0300, Michael S. Tsirkin wrote:> On Tue, Apr 24, 2018 at 09:16:38AM +0800, Tiwei Bie wrote: > > On Tue, Apr 24, 2018 at 04:05:07AM +0300, Michael S. Tsirkin wrote: > > > On Tue, Apr 24, 2018 at 08:54:52AM +0800, Jason Wang wrote: > > > > > > > > > > > > On 2018?04?23? 17:29, Tiwei Bie wrote: > > > > > On Mon, Apr 23, 2018 at 01:42:14PM +0800, Jason Wang wrote: > > > > > > On 2018?04?01? 22:12, Tiwei Bie wrote: > > > > > > > Hello everyone, > > > > > > > > > > > > > > This RFC implements packed ring support for virtio driver. > > > > > > > > > > > > > > The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented > > > > > > > by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html > > > > > > > Minor changes are needed for the vhost code, e.g. to kick the guest. > > > > > > > > > > > > > > TODO: > > > > > > > - Refinements and bug fixes; > > > > > > > - Split into small patches; > > > > > > > - Test indirect descriptor support; > > > > > > > - Test/fix event suppression support; > > > > > > > - Test devices other than net; > > > > > > > > > > > > > > RFC v1 -> RFC v2: > > > > > > > - Add indirect descriptor support - compile test only; > > > > > > > - Add event suppression supprt - compile test only; > > > > > > > - Move vring_packed_init() out of uapi (Jason, MST); > > > > > > > - Merge two loops into one in virtqueue_add_packed() (Jason); > > > > > > > - Split vring_unmap_one() for packed ring and split ring (Jason); > > > > > > > - Avoid using '%' operator (Jason); > > > > > > > - Rename free_head -> next_avail_idx (Jason); > > > > > > > - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason); > > > > > > > - Some other refinements and bug fixes; > > > > > > > > > > > > > > Thanks! > > > > > > > > > > > > > > Signed-off-by: Tiwei Bie <tiwei.bie at intel.com> > > > > > > > --- > > > > > > > drivers/virtio/virtio_ring.c | 1094 +++++++++++++++++++++++++++++------- > > > > > > > include/linux/virtio_ring.h | 8 +- > > > > > > > include/uapi/linux/virtio_config.h | 12 +- > > > > > > > include/uapi/linux/virtio_ring.h | 61 ++ > > > > > > > 4 files changed, 980 insertions(+), 195 deletions(-) > > > > > > > > > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > > > > > index 71458f493cf8..0515dca34d77 100644 > > > > > > > --- a/drivers/virtio/virtio_ring.c > > > > > > > +++ b/drivers/virtio/virtio_ring.c > > > > > > > @@ -58,14 +58,15 @@ > > > > > > [...] > > > > > > > > > > > > > + > > > > > > > + if (vq->indirect) { > > > > > > > + u32 len; > > > > > > > + > > > > > > > + desc = vq->desc_state[head].indir_desc; > > > > > > > + /* Free the indirect table, if any, now that it's unmapped. */ > > > > > > > + if (!desc) > > > > > > > + goto out; > > > > > > > + > > > > > > > + len = virtio32_to_cpu(vq->vq.vdev, > > > > > > > + vq->vring_packed.desc[head].len); > > > > > > > + > > > > > > > + BUG_ON(!(vq->vring_packed.desc[head].flags & > > > > > > > + cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT))); > > > > > > It looks to me spec does not force to keep VRING_DESC_F_INDIRECT here. So we > > > > > > can safely remove this BUG_ON() here. > > > > > > > > > > > > > + BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc)); > > > > > > Len could be ignored for used descriptor according to the spec, so we need > > > > > > remove this BUG_ON() too. > > > > > Yeah, you're right! The BUG_ON() isn't right. I'll remove it. > > > > > And I think something related to this in the spec isn't very > > > > > clear currently. > > > > > > > > > > In the spec, there are below words: > > > > > > > > > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L272 > > > > > """ > > > > > In descriptors with VIRTQ_DESC_F_INDIRECT set VIRTQ_DESC_F_WRITE > > > > > is reserved and is ignored by the device. > > > > > """ > > > > > > > > > > So when device writes back an used descriptor in this case, > > > > > device may not set the VIRTQ_DESC_F_WRITE flag as the flag > > > > > is reserved and should be ignored. > > > > > > > > > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L170 > > > > > """ > > > > > Element Length is reserved for used descriptors without the > > > > > VIRTQ_DESC_F_WRITE flag, and is ignored by drivers. > > > > > """ > > > > > > > > > > And this is the way how driver ignores the `len` in an used > > > > > descriptor. > > > > > > > > > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L241 > > > > > """ > > > > > To increase ring capacity the driver can store a (read-only > > > > > by the device) table of indirect descriptors anywhere in memory, > > > > > and insert a descriptor in the main virtqueue (with \field{Flags} > > > > > bit VIRTQ_DESC_F_INDIRECT on) that refers to a buffer element > > > > > containing this indirect descriptor table; > > > > > """ > > > > > > > > > > So the indirect descriptors in the table are read-only by > > > > > the device. And the only descriptor which is writeable by > > > > > the device is the descriptor in the main virtqueue (with > > > > > Flags bit VIRTQ_DESC_F_INDIRECT on). So if we ignore the > > > > > `len` in this descriptor, we won't be able to get the > > > > > length of the data written by the device. > > > > > > > > > > So I think the `len` in this descriptor will carry the > > > > > length of the data written by the device (if the buffers > > > > > are writable to the device) even if the VIRTQ_DESC_F_WRITE > > > > > isn't set by the device. How do you think? > > > > > > > > Yes I think so. But we'd better need clarification from Michael. > > > > > > I think if you use a descriptor, and you want to supply len > > > to guest, you set VIRTQ_DESC_F_WRITE in the used descriptor. > > > Spec also says you must not set VIRTQ_DESC_F_INDIRECT then. > > > If that's a problem we could look at relaxing that last requirement - > > > does driver want INDIRECT in used descriptor to match > > > the value in the avail descriptor for some reason? > > > > For indirect, driver needs some way to get the length > > of the data written by the driver. And the descriptors > > in the indirect table is read-only, so the only place > > device could put this value is the descriptor with the > > VIRTQ_DESC_F_INDIRECT flag set. > > when writing out used descriptor, device should set VIRTQ_DESC_F_WRITE > (and clear VIRTQ_DESC_F_INDIRECT).So the spec allows device to set VIRTQ_DESC_F_WRITE bit when writing out an used descriptor even if the corresponding descriptors it just parsed don't have the VIRTQ_DESC_F_WRITE bit set? Best regards, Tiwei Bie
On Tue, Apr 24, 2018 at 09:37:47AM +0800, Tiwei Bie wrote:> On Tue, Apr 24, 2018 at 04:29:51AM +0300, Michael S. Tsirkin wrote: > > On Tue, Apr 24, 2018 at 09:16:38AM +0800, Tiwei Bie wrote: > > > On Tue, Apr 24, 2018 at 04:05:07AM +0300, Michael S. Tsirkin wrote: > > > > On Tue, Apr 24, 2018 at 08:54:52AM +0800, Jason Wang wrote: > > > > > > > > > > > > > > > On 2018?04?23? 17:29, Tiwei Bie wrote: > > > > > > On Mon, Apr 23, 2018 at 01:42:14PM +0800, Jason Wang wrote: > > > > > > > On 2018?04?01? 22:12, Tiwei Bie wrote: > > > > > > > > Hello everyone, > > > > > > > > > > > > > > > > This RFC implements packed ring support for virtio driver. > > > > > > > > > > > > > > > > The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented > > > > > > > > by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html > > > > > > > > Minor changes are needed for the vhost code, e.g. to kick the guest. > > > > > > > > > > > > > > > > TODO: > > > > > > > > - Refinements and bug fixes; > > > > > > > > - Split into small patches; > > > > > > > > - Test indirect descriptor support; > > > > > > > > - Test/fix event suppression support; > > > > > > > > - Test devices other than net; > > > > > > > > > > > > > > > > RFC v1 -> RFC v2: > > > > > > > > - Add indirect descriptor support - compile test only; > > > > > > > > - Add event suppression supprt - compile test only; > > > > > > > > - Move vring_packed_init() out of uapi (Jason, MST); > > > > > > > > - Merge two loops into one in virtqueue_add_packed() (Jason); > > > > > > > > - Split vring_unmap_one() for packed ring and split ring (Jason); > > > > > > > > - Avoid using '%' operator (Jason); > > > > > > > > - Rename free_head -> next_avail_idx (Jason); > > > > > > > > - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason); > > > > > > > > - Some other refinements and bug fixes; > > > > > > > > > > > > > > > > Thanks! > > > > > > > > > > > > > > > > Signed-off-by: Tiwei Bie <tiwei.bie at intel.com> > > > > > > > > --- > > > > > > > > drivers/virtio/virtio_ring.c | 1094 +++++++++++++++++++++++++++++------- > > > > > > > > include/linux/virtio_ring.h | 8 +- > > > > > > > > include/uapi/linux/virtio_config.h | 12 +- > > > > > > > > include/uapi/linux/virtio_ring.h | 61 ++ > > > > > > > > 4 files changed, 980 insertions(+), 195 deletions(-) > > > > > > > > > > > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > > > > > > index 71458f493cf8..0515dca34d77 100644 > > > > > > > > --- a/drivers/virtio/virtio_ring.c > > > > > > > > +++ b/drivers/virtio/virtio_ring.c > > > > > > > > @@ -58,14 +58,15 @@ > > > > > > > [...] > > > > > > > > > > > > > > > + > > > > > > > > + if (vq->indirect) { > > > > > > > > + u32 len; > > > > > > > > + > > > > > > > > + desc = vq->desc_state[head].indir_desc; > > > > > > > > + /* Free the indirect table, if any, now that it's unmapped. */ > > > > > > > > + if (!desc) > > > > > > > > + goto out; > > > > > > > > + > > > > > > > > + len = virtio32_to_cpu(vq->vq.vdev, > > > > > > > > + vq->vring_packed.desc[head].len); > > > > > > > > + > > > > > > > > + BUG_ON(!(vq->vring_packed.desc[head].flags & > > > > > > > > + cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT))); > > > > > > > It looks to me spec does not force to keep VRING_DESC_F_INDIRECT here. So we > > > > > > > can safely remove this BUG_ON() here. > > > > > > > > > > > > > > > + BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc)); > > > > > > > Len could be ignored for used descriptor according to the spec, so we need > > > > > > > remove this BUG_ON() too. > > > > > > Yeah, you're right! The BUG_ON() isn't right. I'll remove it. > > > > > > And I think something related to this in the spec isn't very > > > > > > clear currently. > > > > > > > > > > > > In the spec, there are below words: > > > > > > > > > > > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L272 > > > > > > """ > > > > > > In descriptors with VIRTQ_DESC_F_INDIRECT set VIRTQ_DESC_F_WRITE > > > > > > is reserved and is ignored by the device. > > > > > > """ > > > > > > > > > > > > So when device writes back an used descriptor in this case, > > > > > > device may not set the VIRTQ_DESC_F_WRITE flag as the flag > > > > > > is reserved and should be ignored. > > > > > > > > > > > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L170 > > > > > > """ > > > > > > Element Length is reserved for used descriptors without the > > > > > > VIRTQ_DESC_F_WRITE flag, and is ignored by drivers. > > > > > > """ > > > > > > > > > > > > And this is the way how driver ignores the `len` in an used > > > > > > descriptor. > > > > > > > > > > > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L241 > > > > > > """ > > > > > > To increase ring capacity the driver can store a (read-only > > > > > > by the device) table of indirect descriptors anywhere in memory, > > > > > > and insert a descriptor in the main virtqueue (with \field{Flags} > > > > > > bit VIRTQ_DESC_F_INDIRECT on) that refers to a buffer element > > > > > > containing this indirect descriptor table; > > > > > > """ > > > > > > > > > > > > So the indirect descriptors in the table are read-only by > > > > > > the device. And the only descriptor which is writeable by > > > > > > the device is the descriptor in the main virtqueue (with > > > > > > Flags bit VIRTQ_DESC_F_INDIRECT on). So if we ignore the > > > > > > `len` in this descriptor, we won't be able to get the > > > > > > length of the data written by the device. > > > > > > > > > > > > So I think the `len` in this descriptor will carry the > > > > > > length of the data written by the device (if the buffers > > > > > > are writable to the device) even if the VIRTQ_DESC_F_WRITE > > > > > > isn't set by the device. How do you think? > > > > > > > > > > Yes I think so. But we'd better need clarification from Michael. > > > > > > > > I think if you use a descriptor, and you want to supply len > > > > to guest, you set VIRTQ_DESC_F_WRITE in the used descriptor. > > > > Spec also says you must not set VIRTQ_DESC_F_INDIRECT then. > > > > If that's a problem we could look at relaxing that last requirement - > > > > does driver want INDIRECT in used descriptor to match > > > > the value in the avail descriptor for some reason? > > > > > > For indirect, driver needs some way to get the length > > > of the data written by the driver. And the descriptors > > > in the indirect table is read-only, so the only place > > > device could put this value is the descriptor with the > > > VIRTQ_DESC_F_INDIRECT flag set. > > > > when writing out used descriptor, device should set VIRTQ_DESC_F_WRITE > > (and clear VIRTQ_DESC_F_INDIRECT). > > So the spec allows device to set VIRTQ_DESC_F_WRITE bit > when writing out an used descriptor even if the corresponding > descriptors it just parsed don't have the VIRTQ_DESC_F_WRITE > bit set? > > Best regards, > Tiwei BieI think so. In a used descriptor, VIRTQ_DESC_F_WRITE just means length is valid. -- MST