Re: [Qemu-devel] [PATCH v6 1/7] vhost-user: Support transferring inflight buffer between qemu and backend

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

On Sat, Feb 23, 2019 at 09:10:01PM +0800, Yongji Xie wrote:
> On Fri, 22 Feb 2019 at 22:54, Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Fri, Feb 22, 2019 at 03:05:23PM +0800, Yongji Xie wrote:
> > > On Fri, 22 Feb 2019 at 14:21, Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > On Fri, Feb 22, 2019 at 10:47:03AM +0800, Yongji Xie wrote:
> > > > > > > +
> > > > > > > +To track inflight I/O, the queue region should be processed as follows:
> > > > > > > +
> > > > > > > +When receiving available buffers from the driver:
> > > > > > > +
> > > > > > > +    1. Get the next available head-descriptor index from available ring, i
> > > > > > > +
> > > > > > > +    2. Set desc[i].inflight to 1
> > > > > > > +
> > > > > > > +When supplying used buffers to the driver:
> > > > > > > +
> > > > > > > +    1. Get corresponding used head-descriptor index, i
> > > > > > > +
> > > > > > > +    2. Set desc[i].next to process_head
> > > > > > > +
> > > > > > > +    3. Set process_head to i
> > > > > > > +
> > > > > > > +    4. Steps 1,2,3 may be performed repeatedly if batching is possible
> > > > > > > +
> > > > > > > +    5. Increase the idx value of used ring by the size of the batch
> > > > > > > +
> > > > > > > +    6. Set the inflight field of each DescStateSplit entry in the batch to 0
> > > > > > > +
> > > > > > > +    7. Set used_idx to the idx value of used ring
> > > > > > > +
> > > > > > > +When reconnecting:
> > > > > > > +
> > > > > > > +    1. If the value of used_idx does not match the idx value of used ring,
> > > > > > > +
> > > > > > > +        (a) Subtract the value of used_idx from the idx value of used ring to get
> > > > > > > +        the number of in-progress DescStateSplit entries
> > > > > > > +
> > > > > > > +        (b) Set the inflight field of the in-progress DescStateSplit entries which
> > > > > > > +        start from process_head to 0
> > > > > > > +
> > > > > > > +        (c) Set used_idx to the idx value of used ring
> > > > > > > +
> > > > > > > +    2. Resubmit each inflight DescStateSplit entry
> > > > > >
> > > > > > I re-read a couple of time and I still don't understand what it says.
> > > > > >
> > > > > > For simplicity consider split ring. So we want a list of heads that are
> > > > > > outstanding. Fair enough. Now device finishes a head. What now? I needs
> > > > > > to drop head from the list. But list is unidirectional (just next, no
> > > > > > prev). So how can you drop an entry from the middle?
> > > > > >
> > > > >
> > > > > The process_head is only used when slave crash between increasing the
> > > > > idx value of used ring and updating used_idx. We use it to find the
> > > > > in-progress DescStateSplit entries before the crash and complete them
> > > > > when reconnecting. Make sure guest and slave have the same view for
> > > > > inflight I/Os.
> > > > >
> > > >
> > > > But I don't understand how does the described process help do it?
> > > >
> > >
> > > For example, we need to submit descriptors A, B, C to driver in a batch.
> > >
> > > Firstly, we will link those descriptors like:
> > >
> > > process_head->A->B->C    (A)
> > >
> > > Then, we need to update idx value of used vring to mark those
> > > descriptors as used:
> > >
> > > _vring.used->idx += 3    (B)
> > >
> > > At last, clear the inflight field of those descriptors and update
> > > used_idx field:
> > >
> > > A.inflight = 0; B.inflight = 0; C.inflight = 0;    (C)
> > >
> > > used_idx = _vring.used->idx;    (D)
> > >
> > > After (B), guest can consume the descriptors A,B,C. So we must make
> > > sure the inflight field of A,B,C is cleared when reconnecting to avoid
> > > re-submitting used descriptor. If slave crash during (C), the inflight
> > > field of A,B,C may be incorrect. To detect that case, we can see
> > > whether used_idx matches _vring.used->idx. And through process_head,
> > > we can get the in-progress descriptors A,B,C and clear their inflight
> > > field again when reconnecting.
> > >
> > > >
> > > > > In other case, the inflight field is enough to track inflight I/O.
> > > > > When reconnecting, we go through all DescStateSplit entries and
> > > > > re-submit the entry whose inflight field is equal to 1.
> > > >
> > > > What I don't understand is how do we know the order
> > > > in which they have to be resubmitted. Reordering
> > > > operations would be a big problem, won't it?
> > > >
> > >
> > > In previous patch, I record avail_idx for each DescStateSplit entry to
> > > preserve the order. Is it useful to fix this?
> > >
> > > >
> > > > Let's say I fetch descriptors A, B, C and start
> > > > processing. how does memory look?
> > >
> > > A.inflight = 1, C.inflight = 1, B.inflight = 1
> > >
> > > > Now I finished B and marked it used. How does
> > > > memory look?
> > > >
> > >
> > > A.inflight = 1, C.inflight = 1, B.inflight = 0, process_head = B
> >
> > OK. And we have
> >
> > process_head->B->process_head
> >
> > ?
> >
> > Now if there is a reconnect, I want to submit A and then C,
> > correct? How do I know that from this picture? How do I
> > know to start with A? It's not on the list anymore...
> >
> 
> We can go through all DescStateSplit entries (track all descriptors in
> Descriptor Table), then we can find A and C are inflight entry by its
> inflight field. And if we want to resubmit them in order (submit A and
> then C), we need to introduce a timestamp for each DescStateSplit
> entry to preserve the order when we fetch them from driver. Something
> like:
> 
> When receiving available buffers from the driver:
> 
> 1. Get the next available head-descriptor index from available ring, i
> 
> 2. desc[i].timestamp = avail_idx++;
> 
> 3. Set desc[i].inflight to 1
> 
> Thanks,
> Yongji

OK I guess a 64 bit counter would be fine for that.

In order seems critical for storage right?
Reordering write would seem to lead to data corruption.

But now I don't understand what does the next
field do. So it so you can maintain a freelist
within a statically allocated array?

-- 
MST