On 2021/2/9 ??11:35, Eugenio Perez Martin wrote:> On Fri, Feb 5, 2021 at 4:52 AM Jason Wang <jasowang at redhat.com>
wrote:
>>
>> On 2021/2/4 ??5:25, Eugenio Perez Martin wrote:
>>> On Thu, Feb 4, 2021 at 4:14 AM Jason Wang <jasowang at
redhat.com> wrote:
>>>> On 2021/2/2 ??6:17, Eugenio Perez Martin wrote:
>>>>> On Tue, Feb 2, 2021 at 4:31 AM Jason Wang <jasowang at
redhat.com> wrote:
>>>>>> On 2021/2/1 ??4:28, Eugenio Perez Martin wrote:
>>>>>>> On Mon, Feb 1, 2021 at 7:13 AM Jason Wang
<jasowang at redhat.com> wrote:
>>>>>>>> On 2021/1/30 ??4:54, Eugenio P?rez wrote:
>>>>>>>>> Signed-off-by: Eugenio P?rez <eperezma
at redhat.com>
>>>>>>>>> ---
>>>>>>>>> include/hw/virtio/vhost.h | 1 +
>>>>>>>>> hw/virtio/vhost.c | 17
+++++++++++++++++
>>>>>>>>> 2 files changed, 18 insertions(+)
>>>>>>>>>
>>>>>>>>> diff --git a/include/hw/virtio/vhost.h
b/include/hw/virtio/vhost.h
>>>>>>>>> index 4a8bc75415..fca076e3f0 100644
>>>>>>>>> --- a/include/hw/virtio/vhost.h
>>>>>>>>> +++ b/include/hw/virtio/vhost.h
>>>>>>>>> @@ -123,6 +123,7 @@ uint64_t
vhost_get_features(struct vhost_dev *hdev, const int *feature_bits,
>>>>>>>>> void vhost_ack_features(struct
vhost_dev *hdev, const int *feature_bits,
>>>>>>>>> uint64_t
features);
>>>>>>>>> bool vhost_has_free_slot(void);
>>>>>>>>> +struct vhost_dev
*vhost_dev_from_virtio(const VirtIODevice *vdev);
>>>>>>>>>
>>>>>>>>> int vhost_net_set_backend(struct
vhost_dev *hdev,
>>>>>>>>> struct
vhost_vring_file *file);
>>>>>>>>> diff --git a/hw/virtio/vhost.c
b/hw/virtio/vhost.c
>>>>>>>>> index 28c7d78172..8683d507f5 100644
>>>>>>>>> --- a/hw/virtio/vhost.c
>>>>>>>>> +++ b/hw/virtio/vhost.c
>>>>>>>>> @@ -61,6 +61,23 @@ bool
vhost_has_free_slot(void)
>>>>>>>>> return slots_limit >
used_memslots;
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> +/*
>>>>>>>>> + * Get the vhost device associated to a
VirtIO device.
>>>>>>>>> + */
>>>>>>>>> +struct vhost_dev
*vhost_dev_from_virtio(const VirtIODevice *vdev)
>>>>>>>>> +{
>>>>>>>>> + struct vhost_dev *hdev;
>>>>>>>>> +
>>>>>>>>> + QLIST_FOREACH(hdev,
&vhost_devices, entry) {
>>>>>>>>> + if (hdev->vdev == vdev) {
>>>>>>>>> + return hdev;
>>>>>>>>> + }
>>>>>>>>> + }
>>>>>>>>> +
>>>>>>>>> + assert(hdev);
>>>>>>>>> + return NULL;
>>>>>>>>> +}
>>>>>>>> I'm not sure this can work in the case of
multiqueue. E.g vhost-net
>>>>>>>> multiqueue is a N:1 mapping between vhost
devics and virtio devices.
>>>>>>>>
>>>>>>>> Thanks
>>>>>>>>
>>>>>>> Right. We could add an "vdev vq index"
parameter to the function in
>>>>>>> this case, but I guess the most reliable way to do
this is to add a
>>>>>>> vhost_opaque value to VirtQueue, as Stefan proposed
in previous RFC.
>>>>>> So the question still, it looks like it's easier to
hide the shadow
>>>>>> virtqueue stuffs at vhost layer instead of expose them
to virtio layer:
>>>>>>
>>>>>> 1) vhost protocol is stable ABI
>>>>>> 2) no need to deal with virtio stuffs which is more
complex than vhost
>>>>>>
>>>>>> Or are there any advantages if we do it at virtio
layer?
>>>>>>
>>>>> As far as I can tell, we will need the virtio layer the
moment we
>>>>> start copying/translating buffers.
>>>>>
>>>>> In this series, the virtio dependency can be reduced if
qemu does not
>>>>> check the used ring _F_NO_NOTIFY flag before writing to
irqfd. It
>>>>> would enable packed queues and IOMMU immediately, and I
think the cost
>>>>> should not be so high. In the previous RFC this check was
deleted
>>>>> later anyway, so I think it was a bad idea to include it
from the start.
>>>> I am not sure I understand here. For vhost, we can still do
anything we
>>>> want, e.g accessing guest memory etc. Any blocker that prevent
us from
>>>> copying/translating buffers? (Note that qemu will propagate
memory
>>>> mappings to vhost).
>>>>
>>> There is nothing that forbids us to access directly, but if we
don't
>>> reuse the virtio layer functionality we would have to duplicate
every
>>> access function. "Need" was a too strong word maybe :).
>>>
>>> In other words: for the shadow vq vring exposed for the device,
qemu
>>> treats it as a driver, and this functionality needs to be added to
>>> qemu. But for accessing the guest's one do not reuse virtio.c
would be
>>> a bad idea in my opinion.
>>
>> The problem is, virtio.c is not a library and it has a lot of
dependency
>> with other qemu modules basically makes it impossible to be reused at
>> vhost level.
>>
> While virtio.c as a whole has dependencies, I think that the functions
> needed in the original RFC do not have these dependencies.
>
> However I see how to split vring dataplane from virtio device
> management can benefit.
If you can split them out, that would be fine.
>
>> We can solve this by:
>>
>> 1) split the core functions out as a library or
>> 2) switch to use contrib/lib-vhostuser but needs to decouple UNIX
socket
>> transport
>>
>> None of the above looks trivial and they are only device codes. For
>> shadow virtqueue, we need driver codes as well where no code can be
reused.
>>
>> As we discussed, we probably need IOVA allocated when forwarding
>> descriptors between the two virtqueues. So my feeling is we can have
our
>> own codes to start then we can consider whether we can reuse some from
>> the existing virtio.c or lib-vhostuser.
>>
> As I see it, if we develop our own code a lot of it will be copied
> from current virtio.c, which itself duplicates a lot of contrib/ lib
> functionality.
>
> Maybe it's better to combine your proposals and decouple the vring
> functions, the vhost transport, and the qemu virtio device management,
> so other projects can reuse them directly?
I think this can work.
>
> I still think this can be left for a later series with buffer
> forwarding on top of this one, do you think they can/should be merged
> independently?
Since you post a new series, let's see.
Thanks
>
> Thanks!
>
>> Thanks
>>
>>
>>>> Thanks
>>>>
>>>>
>>>>>
>>>>>
>>>>>
>>>>>> Thanks
>>>>>>
>>>>>>
>>>>>>> I need to take this into account in
qmp_x_vhost_enable_shadow_vq too.
>>>>>>>
>>>>>>>>> +
>>>>>>>>> static void
vhost_dev_sync_region(struct vhost_dev *dev,
>>>>>>>>>
MemoryRegionSection *section,
>>>>>>>>>
uint64_t mfirst, uint64_t mlast,