Jason Wang
2021-Feb-24 09:29 UTC
[virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
On 2021/2/23 6:58 ??, Cornelia Huck wrote:> On Tue, 23 Feb 2021 18:31:07 +0800 > Jason Wang <jasowang at redhat.com> wrote: > >> On 2021/2/23 6:04 ??, Cornelia Huck wrote: >>> On Tue, 23 Feb 2021 17:46:20 +0800 >>> Jason Wang <jasowang at redhat.com> wrote: >>> >>>> On 2021/2/23 ??5:25, Michael S. Tsirkin wrote: >>>>> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote: >>>>>> On 2/21/2021 8:14 PM, Jason Wang wrote: >>>>>>> On 2021/2/19 7:54 ??, Si-Wei Liu wrote: >>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked >>>>>>>> for legacy") made an exception for legacy guests to reset >>>>>>>> features to 0, when config space is accessed before features >>>>>>>> are set. We should relieve the verify_min_features() check >>>>>>>> and allow features reset to 0 for this case. >>>>>>>> >>>>>>>> It's worth noting that not just legacy guests could access >>>>>>>> config space before features are set. For instance, when >>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver >>>>>>>> will try to access and validate the MTU present in the config >>>>>>>> space before virtio features are set. >>>>>>> This looks like a spec violation: >>>>>>> >>>>>>> " >>>>>>> >>>>>>> The following driver-read-only field, mtu only exists if >>>>>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the >>>>>>> driver to use. >>>>>>> " >>>>>>> >>>>>>> Do we really want to workaround this? >>>>>> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest? >>>>>> >>>>>> I think the point is, since there's legacy guest we'd have to support, this >>>>>> host side workaround is unavoidable. Although I agree the violating driver >>>>>> should be fixed (yes, it's in today's upstream kernel which exists for a >>>>>> while now). >>>>> Oh you are right: >>>>> >>>>> >>>>> static int virtnet_validate(struct virtio_device *vdev) >>>>> { >>>>> if (!vdev->config->get) { >>>>> dev_err(&vdev->dev, "%s failure: config access disabled\n", >>>>> __func__); >>>>> return -EINVAL; >>>>> } >>>>> >>>>> if (!virtnet_validate_features(vdev)) >>>>> return -EINVAL; >>>>> >>>>> if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) { >>>>> int mtu = virtio_cread16(vdev, >>>>> offsetof(struct virtio_net_config, >>>>> mtu)); >>>>> if (mtu < MIN_MTU) >>>>> __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU); >>>> I wonder why not simply fail here? >>> I think both failing or not accepting the feature can be argued to make >>> sense: "the device presented us with a mtu size that does not make >>> sense" would point to failing, "we cannot work with the mtu size that >>> the device presented us" would point to not negotiating the feature. >>> >>>> >>>>> } >>>>> >>>>> return 0; >>>>> } >>>>> >>>>> And the spec says: >>>>> >>>>> >>>>> The driver MUST follow this sequence to initialize a device: >>>>> 1. Reset the device. >>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device. >>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the device. >>>>> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the >>>>> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration >>>>> fields to check that it can support the device before accepting it. >>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step. >>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not >>>>> support our subset of features and the device is unusable. >>>>> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup, >>>>> reading and possibly writing the device?s virtio configuration space, and population of virtqueues. >>>>> 8. Set the DRIVER_OK status bit. At this point the device is ?live?. >>>>> >>>>> >>>>> Item 4 on the list explicitly allows reading config space before >>>>> FEATURES_OK. >>>>> >>>>> I conclude that VIRTIO_NET_F_MTU is set means "set in device features". >>>> So this probably need some clarification. "is set" is used many times in >>>> the spec that has different implications. >>> Before FEATURES_OK is set by the driver, I guess it means "the device >>> has offered the feature"; >> >> For me this part is ok since it clarify that it's the driver that set >> the bit. >> >> >> >>> during normal usage, it means "the feature >>> has been negotiated". >> /? >> >> It looks to me the feature negotiation is done only after device set >> FEATURES_OK, or FEATURES_OK could be read from device status? > I'd consider feature negotiation done when the driver reads FEATURES_OK > back from the status.I agree.> >> >>> (This is a bit fuzzy for legacy mode.) > ...because legacy does not have FEATURES_OK. > >> >> The problem is the MTU description for example: >> >> "The following driver-read-only field, mtu only exists if >> VIRTIO_NET_F_MTU is set." >> >> It looks to me need to use "if VIRTIO_NET_F_MTU is set by device". > "offered by the device"? I don't think it should 'disappear' from the > config space if the driver won't use it. (Same for other config space > fields that are tied to feature bits.)But what happens if e.g device doesn't offer VIRTIO_NET_F_MTU? It looks to according to the spec there will be no mtu field. And a more interesting case is VIRTIO_NET_F_MQ is not offered but VIRTIO_NET_F_MTU offered. To me, it means we don't have max_virtqueue_pairs but it's not how the driver is wrote today.> >> Otherwise readers (at least for me), may think the MTU is only valid >> if driver set the bit. > I think it would still be 'valid' in the sense that it exists and has > some value in there filled in by the device, but a driver reading it > without negotiating the feature would be buggy. (Like in the kernel > code above; the kernel not liking the value does not make the field > invalid.)See Michael's reply, the spec allows read the config before setting features.> > Maybe a statement covering everything would be: > > "The following driver-read-only field mtu only exists if the device > offers VIRTIO_NET_F_MTU and may be read by the driver during feature > negotiation and after VIRTIO_NET_F_MTU has been successfully > negotiated." > >> >>> Should we add a wording clarification to the spec? >> >> I think so. > Some clarification would be needed for each field that depends on a > feature; that would be quite verbose. Maybe we can get away with a > clarifying statement? > > "Some config space fields may depend on a certain feature. In that > case, the field exits if the device has offered the corresponding > feature,So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config will look like: struct virtio_net_config { ??????? u8 mac[6]; ??????? le16 status; ??????? le16 mtu; };> and may be read by the driver during feature negotiation, and > accessed by the driver after the feature has been successfully > negotiated. A shorthand for this is a statement that a field only > exists if a certain feature bit is set."I'm not sure using "shorthand" is good for the spec, at least we can limit the its scope only to the configuration space part. Thanks>
Cornelia Huck
2021-Feb-24 11:12 UTC
[virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
On Wed, 24 Feb 2021 17:29:07 +0800 Jason Wang <jasowang at redhat.com> wrote:> On 2021/2/23 6:58 ??, Cornelia Huck wrote: > > On Tue, 23 Feb 2021 18:31:07 +0800 > > Jason Wang <jasowang at redhat.com> wrote: > > > >> On 2021/2/23 6:04 ??, Cornelia Huck wrote: > >>> On Tue, 23 Feb 2021 17:46:20 +0800 > >>> Jason Wang <jasowang at redhat.com> wrote: > >>> > >>>> On 2021/2/23 ??5:25, Michael S. Tsirkin wrote: > >>>>> On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote: > >>>>>> On 2/21/2021 8:14 PM, Jason Wang wrote: > >>>>>>> On 2021/2/19 7:54 ??, Si-Wei Liu wrote: > >>>>>>>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked > >>>>>>>> for legacy") made an exception for legacy guests to reset > >>>>>>>> features to 0, when config space is accessed before features > >>>>>>>> are set. We should relieve the verify_min_features() check > >>>>>>>> and allow features reset to 0 for this case. > >>>>>>>> > >>>>>>>> It's worth noting that not just legacy guests could access > >>>>>>>> config space before features are set. For instance, when > >>>>>>>> feature VIRTIO_NET_F_MTU is advertised some modern driver > >>>>>>>> will try to access and validate the MTU present in the config > >>>>>>>> space before virtio features are set. > >>>>>>> This looks like a spec violation: > >>>>>>> > >>>>>>> " > >>>>>>> > >>>>>>> The following driver-read-only field, mtu only exists if > >>>>>>> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the > >>>>>>> driver to use. > >>>>>>> " > >>>>>>> > >>>>>>> Do we really want to workaround this? > >>>>>> Isn't the commit 452639a64ad8 itself is a workaround for legacy guest? > >>>>>> > >>>>>> I think the point is, since there's legacy guest we'd have to support, this > >>>>>> host side workaround is unavoidable. Although I agree the violating driver > >>>>>> should be fixed (yes, it's in today's upstream kernel which exists for a > >>>>>> while now). > >>>>> Oh you are right: > >>>>> > >>>>> > >>>>> static int virtnet_validate(struct virtio_device *vdev) > >>>>> { > >>>>> if (!vdev->config->get) { > >>>>> dev_err(&vdev->dev, "%s failure: config access disabled\n", > >>>>> __func__); > >>>>> return -EINVAL; > >>>>> } > >>>>> > >>>>> if (!virtnet_validate_features(vdev)) > >>>>> return -EINVAL; > >>>>> > >>>>> if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) { > >>>>> int mtu = virtio_cread16(vdev, > >>>>> offsetof(struct virtio_net_config, > >>>>> mtu)); > >>>>> if (mtu < MIN_MTU) > >>>>> __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU); > >>>> I wonder why not simply fail here? > >>> I think both failing or not accepting the feature can be argued to make > >>> sense: "the device presented us with a mtu size that does not make > >>> sense" would point to failing, "we cannot work with the mtu size that > >>> the device presented us" would point to not negotiating the feature. > >>> > >>>> > >>>>> } > >>>>> > >>>>> return 0; > >>>>> } > >>>>> > >>>>> And the spec says: > >>>>> > >>>>> > >>>>> The driver MUST follow this sequence to initialize a device: > >>>>> 1. Reset the device. > >>>>> 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device. > >>>>> 3. Set the DRIVER status bit: the guest OS knows how to drive the device. > >>>>> 4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the > >>>>> device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration > >>>>> fields to check that it can support the device before accepting it. > >>>>> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step. > >>>>> 6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not > >>>>> support our subset of features and the device is unusable. > >>>>> 7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup, > >>>>> reading and possibly writing the device?s virtio configuration space, and population of virtqueues. > >>>>> 8. Set the DRIVER_OK status bit. At this point the device is ?live?. > >>>>> > >>>>> > >>>>> Item 4 on the list explicitly allows reading config space before > >>>>> FEATURES_OK. > >>>>> > >>>>> I conclude that VIRTIO_NET_F_MTU is set means "set in device features". > >>>> So this probably need some clarification. "is set" is used many times in > >>>> the spec that has different implications. > >>> Before FEATURES_OK is set by the driver, I guess it means "the device > >>> has offered the feature"; > >> > >> For me this part is ok since it clarify that it's the driver that set > >> the bit. > >> > >> > >> > >>> during normal usage, it means "the feature > >>> has been negotiated". > >> /? > >> > >> It looks to me the feature negotiation is done only after device set > >> FEATURES_OK, or FEATURES_OK could be read from device status? > > I'd consider feature negotiation done when the driver reads FEATURES_OK > > back from the status. > > > I agree. > > > > > >> > >>> (This is a bit fuzzy for legacy mode.) > > ...because legacy does not have FEATURES_OK. > > > >> > >> The problem is the MTU description for example: > >> > >> "The following driver-read-only field, mtu only exists if > >> VIRTIO_NET_F_MTU is set." > >> > >> It looks to me need to use "if VIRTIO_NET_F_MTU is set by device". > > "offered by the device"? I don't think it should 'disappear' from the > > config space if the driver won't use it. (Same for other config space > > fields that are tied to feature bits.) > > > But what happens if e.g device doesn't offer VIRTIO_NET_F_MTU? It looks > to according to the spec there will be no mtu field.I think so, yes.> > And a more interesting case is VIRTIO_NET_F_MQ is not offered but > VIRTIO_NET_F_MTU offered. To me, it means we don't have > max_virtqueue_pairs but it's not how the driver is wrote today.That would be a bug, but it seems to me that the virtio-net driver reads max_virtqueue_pairs conditionally and handles absence of the feature correctly?> > > > > >> Otherwise readers (at least for me), may think the MTU is only valid > >> if driver set the bit. > > I think it would still be 'valid' in the sense that it exists and has > > some value in there filled in by the device, but a driver reading it > > without negotiating the feature would be buggy. (Like in the kernel > > code above; the kernel not liking the value does not make the field > > invalid.) > > > See Michael's reply, the spec allows read the config before setting > features.Yes, the period prior to finishing negotiation is obviously special.> > > > > > Maybe a statement covering everything would be: > > > > "The following driver-read-only field mtu only exists if the device > > offers VIRTIO_NET_F_MTU and may be read by the driver during feature > > negotiation and after VIRTIO_NET_F_MTU has been successfully > > negotiated." > > > >> > >>> Should we add a wording clarification to the spec? > >> > >> I think so. > > Some clarification would be needed for each field that depends on a > > feature; that would be quite verbose. Maybe we can get away with a > > clarifying statement? > > > > "Some config space fields may depend on a certain feature. In that > > case, the field exits if the device has offered the corresponding > > feature, > > > So this implies for !VIRTIO_NET_F_MQ && VIRTIO_NET_F_MTU, the config > will look like: > > struct virtio_net_config { > ??????? u8 mac[6]; > ??????? le16 status; > ??????? le16 mtu; > }; >I agree.> > > and may be read by the driver during feature negotiation, and > > accessed by the driver after the feature has been successfully > > negotiated. A shorthand for this is a statement that a field only > > exists if a certain feature bit is set." > > > I'm not sure using "shorthand" is good for the spec, at least we can > limit the its scope only to the configuration space part.Maybe "a shorthand expression"?