Re: [Qemu-devel] [PATCH v5 1/6] vhost-user: Support transferring inflight buffer between qemu and backend

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jan 30, 2019 at 11:49:56AM +0800, Yongji Xie wrote:
> On Wed, 30 Jan 2019 at 10:30, Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Wed, Jan 30, 2019 at 10:07:28AM +0800, Yongji Xie wrote:
> > > On Tue, 29 Jan 2019 at 22:15, Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > On Tue, Jan 29, 2019 at 02:15:35PM +0800, Yongji Xie wrote:
> > > > > On Tue, 29 Jan 2019 at 12:26, Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > > >
> > > > > > On Tue, Jan 29, 2019 at 12:11:55PM +0800, Stefan Hajnoczi wrote:
> > > > > > > On Tue, Jan 22, 2019 at 04:31:47PM +0800, elohimes@gmail.com wrote:
> > > > > > > > +typedef struct DescState {
> > > > > > > > +    uint8_t inuse;
> > > > > > > > +    uint8_t version;
> > > > > > > > +    uint16_t used_idx;
> > > > > > > > +    uint16_t avail_idx;
> > > > > > > > +    uint16_t reserved;
> > > > > > > > +} DescState;
> > > > > > > > +
> > > > > > > > +typedef struct QueueRegion {
> > > > > > > > +    uint8_t valid;
> > > > > >
> > > > > > what's this?
> > > > > >
> > > > >
> > > > > We can use this to check whether this buffer is reset by qemu.
> > > >
> > > >
> > > > I'd use version here. Document that it's 1 currently.
> > >
> > > If we put version into the shared buffer, QEMU will reset it when vm
> > > reset. Then if backend restart at the same time, the version of this
> > > buffer will be lost. So I still let QEMU maintain it and send it back
> > > through message's payload.
> >
> > I don't get it. If it's reset there's no contents.
> > If there's no contents then who cares what the version is?
> >
> 
> What I thought before is that we should not update the buffer when vm
> reset. But now seems like it's unnecessary. I will update this patch
> as you said. Thank you!
> 
> > > > Or do we want feature flags so we can support downgrades too?
> > > >
> > >
> > > We faced the same problem that the feature flags will be lost if QEMU
> > > do not maintain it. So maybe we should put this into message's
> > > payload?
> >
> > I don't understand why we care. We only maintain inflight so we
> > don't need to reset the device. if device is reset we don't
> > need to maintain them at all.
> >
> > > >
> > > > > > > > +    uint16_t desc_num;
> > > > > >
> > > > > > there's padding before this field. Pls make it explicit.
> > > > > >
> > > > >
> > > > > Will do it.
> > > > >
> > > > > > > > +    DescState desc[0];
> > > > > > > > +} QueueRegion;
> > > > > > > > +
> > > > > > > > +The struct DescState is used to describe one head-descriptor's state. The
> > > > > > > > +fields have following meanings:
> > > > > > > > +
> > > > > > > > +    inuse: Indicate whether the descriptor is inuse or not.
> > > > > >
> > > > > > inuse by what?
> > > > > >
> > > > >
> > > > > Maybe inflight is better?
> > > > >
> > > > > > > > +
> > > > > > > > +    version: Indicate whether we have an atomic update to used ring and
> > > > > > > > +    inflight buffer when slave crash at that point. This field should be
> > > > > > > > +    increased by one before and after this two updates. An odd version
> > > > > > > > +    indicates an in-progress update.
> > > > > >
> > > > > > I'm not sure I understand what does the above say. Also does this
> > > > > > require two atomics? Seems pretty expensive. And why is it called
> > > > > > version?
> > > > > >
> > > > > > > > +
> > > > > > > > +    used_idx: Store old index of used ring before we update used ring and
> > > > > > > > +    inflight buffer so that slave can know whether an odd version inflight
> > > > > > > > +    head-descriptor in inflight buffer is processed or not.
> > > > > >
> > > > > > Here too.
> > > > > >
> > > > >
> > > > > Sorry, the above description may be not clear. This two fields are
> > > > > used to indicate whether we have an in-progress update to used ring
> > > > > and inflight buffer. If slave crash before the update to used_ring and
> > > > > after the update to inflight buffer, the version should be odd and
> > > > > used_idx should be equal to used_ring.idx. Then we need to roll back
> > > > > the update to inflight buffer. As for the name of the version filed,
> > > > > actually I didn't find a good one, so I just copy it from struct
> > > > > kvm_steal_time...
> > > > >
> > > > > > > > +
> > > > > > > > +    avail_idx: Used to preserve the descriptor's order in avail ring so that
> > > > > > > > +    slave can resubmit descriptors in order.
> > > > > >
> > > > > > Why would that be necessary?
> > > > > >
> > > > >
> > > > > Maybe some devices will be able to use it to preserve order after
> > > > > reconnecting in future?
> > > >
> > > > If buffers are used in order then old entries in the ring
> > > > are not overwritten so inflight tracking is not
> > > > necessary. This is exactly the case with vhost user net today.
> > > >
> > >
> > > OK, looks reasonable to me.
> > >
> > > > > > >
> > > > > > > Will a completely new "packed vring" inflight shm layout be necessary to
> > > > > > > support the packed vring layout in VIRTIO 1.1?
> > > > > > >
> > > > > > > https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html#x1-610007
> > > > > >
> > > > > > Probably.
> > > > > >
> > > > >
> > > > > How about supporting packed virtqueue in guest driver?
> > > > >
> > > > > Thanks,
> > > > > Yongji
> > > >
> > > > Depends on the guest right? Linux has it:
> > > >
> > >
> > > Sorry, actually I mean I prefer to support inflight tracking for
> > > packed virtqueue in guest driver. This feature is only used by legacy
> > > virtio 1.0 or virtio 0.9 device. What do you think about it?
> > >
> > > Thanks,
> > > Yongji
> >
> > I don't see what does one have to do with the other.  Either we do or we
> > don't want to do it without downtime and retries. If we don't mind
> > resets then let's do it by making guest changes.
> >
> 
> OK, I see. But seems like now we don't support packed virtqueue in
> qemu. Is it better to do that in a splited patchset? Add packed
> virtqueue implenment in contrib/libvhost-user.c, then document the
> inflight I/O tracking behavior.
> 
> Thanks,
> Yongji

it's just documentation so it does not have to depend
on actual code.