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

[Yongji Xie]

On Fri, 22 Feb 2019 at 14:21, Michael S. Tsirkin <address@hidden> 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

> I also wonder how do you address a crash between
> marking descriptor used and clearing inflight.
> Will you redo the descriptor? Is it always safe?
> What if it's a write?
>

It's safe. We can get the in-progess descriptors through process_head
and clear their inflight field when reconnecting.

Thanks,
Yongji