Re: [PATCH v2 00/15] virtio-gpio and various virtio cleanups

[Alex Bennée]

Stefan Hajnoczi <stefanha@gmail.com> writes:

> On Thu, 7 Jul 2022 at 14:42, Alex Bennée <alex.bennee@linaro.org> wrote:
>>
>>
>> Stefan Hajnoczi <stefanha@redhat.com> writes:
>>
>> > [[PGP Signed Part:Undecided]]
>> > On Tue, May 24, 2022 at 04:40:41PM +0100, Alex Bennée wrote:
>> >> Hi,
>> >>
>> >> This series ostensibly adds virtio-user-gpio stubs to the build for
>> >> use with an external vhost-user daemon. We've been testing it with our
>> >> rust daemons from:
>> >>
>> >>   https://github.com/rust-vmm/vhost-device
>> >>
>> >> Getting the test enabled took some doing most likely because the need
>> >> for CONFIG support exercised additional paths in the code that were
>> >> not used for the simpler virtio-net tests. As a result the series has
>> >> a number of cleanup and documentation patches.
>> >>
>> >> The final thing that needed fixing was the ensuring that
>> >> VHOST_USER_F_PROTOCOL_FEATURES didn't get squashed in the negotiation
>> >> process. This was the hardest thing to track down as we store the
>> >> feature bits in several places variously as:
>> >>
>> >>   in VirtIODevice as:
>> >>     uint64_t guest_features;
>> >>     uint64_t host_features;
>> >>     uint64_t backend_features;
>> >
>> > None of these know about VHOST_USER_F_PROTOCOL_FEATURES and vhost-user's
>> > unfiltered feature bits should never be passed to VirtIODevice.
>> >
>> >>
>> >>  in vhost_dev as:
>> >>     uint64_t features;
>> >>     uint64_t acked_features;
>> >>     uint64_t backend_features;
>> >
>> > I don't think these should know about VHOST_USER_F_PROTOCOL_FEATURES
>> > either. AFAIK vhost_dev deals with VIRTIO feature bits, not raw
>> > vhost-user GET_FEATURES.
>>
>> So where does VHOST_USER_F_PROTOCOL_FEATURES get set before it's set
>> with the VHOST_USER_SET_FEATURES message? Currently it's fed via:
>>
>>     uint64_t features = vhost_dev->acked_features;
>>
>> in vhost_dev_set_features() which does apply a few extra bits
>> (VHOST_F_LOG_ALL/VIRTIO_F_IOMMU_PLATFORM). Maybe it should be adding
>> VHOST_USER_F_PROTOCOL_FEATURES here? How should it be signalled by the
>> vhost-user backend that this should be done? Overload the function?
>
> A modern vhost-user server replies to VHOST_USER_GET_FEATURES with
> VHOST_USER_F_PROTOCOL_FEATURES set. That's when the vhost-user client
> encounters this bit.
>
> The vhost-user client should then filter out
> VHOST_USER_F_PROTOCOL_FEATURES because it belongs to the vhost-user
> protocol and isn't a real VIRTIO feature bit. The client uses the
> filtered VIRTIO feature bits and it now knows whether the vhost-user
> server supports the VHOST_USER_GET_PROTOCOL_FEATURES and
> VHOST_USER_SET_PROTOCOL_FEATURES messages.
>
> I think vhost_user_set_features() should set
> VHOST_USER_F_PROTOCOL_FEATURES if the server returned it from
> VHOST_USER_GET_FEATURES. At the moment vhost_user_backend_init()
> stores VHOST_USER_F_PROTOCOL_FEATURES in struct
> vhost_dev->backend_features, which only seems to be used by vhost-net
> code.

I can clean-up the documentation for this. I wonder if the VirtIODevice
backend_features is a duplication that should be removed?

> The other vhost-user devices set acked_features = guest_features and
> ignore backend_features. As a result I guess they don't set
> VHOST_USER_F_PROTOCOL_FEATURES in the VHOST_USER_SET_FEATURES message.
> Most vhost-user servers probably don't care and still respond to
> VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES
> messages (although the vhost-user protocol spec mentions other
> protocol differences when VHOST_USER_F_PROTOCOL_FEATURES is not
> negotiated).
>
> Does this match what you've found? The code is a maze so I may have
> gotten something wrong.

I think so. As you say it's a bit of a maze and hopefully we can more
clearly document when and where things are and how they should be used.
The various virtio devices have grown organically so there are
inconsistencies that need ironing out. 

> In general I think hw/virtio/vhost-user.c
> should be responsible for VHOST_USER_F_PROTOCOL_FEATURES and no other
> part of the QEMU codebase should ever see the bit since it's a
> vhost-user protocol detail and not part of VIRTIO or even a common
> part of vhost.

OK I'll see what I can cook up.

>
> Stefan


-- 
Alex Bennée