Re: [PATCH 05/21] virtio: modify save/load handler to handle inuse varialble.

Yoshiaki Tamura
2010-12-16T07:36:19Z

2010/12/3 Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>:
> 2010/12/2 Michael S. Tsirkin <mst@redhat.com>:
>> On Wed, Dec 01, 2010 at 05:03:43PM +0900, Yoshiaki Tamura wrote:
>>> 2010/11/28 Michael S. Tsirkin <mst@redhat.com>:
>>> > On Sun, Nov 28, 2010 at 08:27:58PM +0900, Yoshiaki Tamura wrote:
>>> >> 2010/11/28 Michael S. Tsirkin <mst@redhat.com>:
>>> >> > On Thu, Nov 25, 2010 at 03:06:44PM +0900, Yoshiaki Tamura wrote:
>>> >> >> Modify inuse type to uint16_t, let save/load to handle, and revert
>>> >> >> last_avail_idx with inuse if there are outstanding emulation.
>>> >> >>
>>> >> >> Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
>>> >> >
>>> >> > This changes migration format, so it will break compatibility with
>>> >> > existing drivers. More generally, I think migrating internal
>>> >> > state that is not guest visible is always a mistake
>>> >> > as it ties migration format to an internal implementation
>>> >> > (yes, I know we do this sometimes, but we should at least
>>> >> > try not to add such cases).  I think the right thing to do in this case
>>> >> > is to flush outstanding
>>> >> > work when vm is stopped.  Then, we are guaranteed that inuse is 0.
>>> >> > I sent patches that do this for virtio net and block.
>>> >>
>>> >> Could you give me the link of your patches?  I'd like to test
>>> >> whether they work with Kemari upon failover.  If they do, I'm
>>> >> happy to drop this patch.
>>> >>
>>> >> Yoshi
>>> >
>>> > Look for this:
>>> > stable migration image on a stopped vm
>>> > sent on:
>>> > Wed, 24 Nov 2010 17:52:49 +0200
>>>
>>> Thanks for the info.
>>>
>>> However, The patch series above didn't solve the issue.  In
>>> case of Kemari, inuse is mostly > 0 because it queues the
>>> output, and while last_avail_idx gets incremented
>>> immediately, not sending inuse makes the state inconsistent
>>> between Primary and Secondary.
>>
>> Hmm. Can we simply avoid incrementing last_avail_idx?
>
> I think we can calculate or prepare an internal last_avail_idx,
> and update the external when inuse is decremented.  I'll try
> whether it work w/ w/o Kemari.

Hi Michael,

Could you please take a look at the following patch?

commit 36ee7910059e6b236fe9467a609f5b4aed866912
Author: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
Date: Thu Dec 16 14:50:54 2010 +0900

virtio: update last_avail_idx when inuse is decreased.

Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>

diff --git a/hw/virtio.c b/hw/virtio.c
index c8a0fc6..6688c02 100644
--- a/hw/virtio.c
+++ b/hw/virtio.c
@@ -237,6 +237,7 @@ void virtqueue_flush(VirtQueue *vq, unsigned int count)
wmb();
trace_virtqueue_flush(vq, count);
vring_used_idx_increment(vq, count);
+ vq->last_avail_idx += count;
vq->inuse -= count;
}

@@ -385,7 +386,7 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)
unsigned int i, head, max;
target_phys_addr_t desc_pa = vq->vring.desc;

- if (!virtqueue_num_heads(vq, vq->last_avail_idx))
+ if (!virtqueue_num_heads(vq, vq->last_avail_idx + vq->inuse))
return 0;

/* When we start there are none of either input nor output. */
@@ -393,7 +394,7 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)

max = vq->vring.num;

- i = head = virtqueue_get_head(vq, vq->last_avail_idx++);
+ i = head = virtqueue_get_head(vq, vq->last_avail_idx + vq->inuse);

if (vring_desc_flags(desc_pa, i) & VRING_DESC_F_INDIRECT) {
if (vring_desc_len(desc_pa, i) % sizeof(VRingDesc)) {

>
>>
>>>  I'm wondering why
>>> last_avail_idx is OK to send but not inuse.
>>
>> last_avail_idx is at some level a mistake, it exposes part of
>> our internal implementation, but it does *also* express
>> a guest observable state.
>>
>> Here's the problem that it solves: just looking at the rings in virtio
>> there is no way to detect that a specific request has already been
>> completed. And the protocol forbids completing the same request twice.
>>
>> Our implementation always starts processing the requests
>> in order, and since we flush outstanding requests
>> before save, it works to just tell the remote 'process only requests
>> after this place'.
>>
>> But there's no such requirement in the virtio protocol,
>> so to be really generic we could add a bitmask of valid avail
>> ring entries that did not complete yet. This would be
>> the exact representation of the guest observable state.
>> In practice we have rings of up to 512 entries.
>> That's 64 byte per ring, not a lot at all.
>>
>> However, if we ever do change the protocol to send the bitmask,
>> we would need some code to resubmit requests
>> out of order, so it's not trivial.
>>
>> Another minor mistake with last_avail_idx is that it has
>> some redundancy: the high bits in the index
>> (> vq size) are not necessary as they can be
>> got from avail idx.  There's a consistency check
>> in load but we really should try to use formats
>> that are always consistent.
>>
>>> The following patch does the same thing as original, yet
>>> keeps the format of the virtio.  It shouldn't break live
>>> migration either because inuse should be 0.
>>>
>>> Yoshi
>>
>> Question is, can you flush to make inuse 0 in kemari too?
>> And if not, how do you handle the fact that some requests
>> are in flight on the primary?
>
> Although we try flushing requests one by one making inuse 0,
> there are cases when it failovers to the secondary when inuse
> isn't 0.  We handle these in flight request on the primary by
> replaying on the secondary.
>
>>
>>> diff --git a/hw/virtio.c b/hw/virtio.c
>>> index c8a0fc6..875c7ca 100644
>>> --- a/hw/virtio.c
>>> +++ b/hw/virtio.c
>>> @@ -664,12 +664,16 @@ void virtio_save(VirtIODevice *vdev, QEMUFile *f)
>>>      qemu_put_be32(f, i);
>>>
>>>      for (i = 0; i < VIRTIO_PCI_QUEUE_MAX; i++) {
>>> +        uint16_t last_avail_idx;
>>> +
>>>          if (vdev->vq[i].vring.num == 0)
>>>              break;
>>>
>>> +        last_avail_idx = vdev->vq[i].last_avail_idx - vdev->vq[i].inuse;
>>> +
>>>          qemu_put_be32(f, vdev->vq[i].vring.num);
>>>          qemu_put_be64(f, vdev->vq[i].pa);
>>> -        qemu_put_be16s(f, &vdev->vq[i].last_avail_idx);
>>> +        qemu_put_be16s(f, &last_avail_idx);
>>>          if (vdev->binding->save_queue)
>>>              vdev->binding->save_queue(vdev->binding_opaque, i, f);
>>>      }
>>>
>>>
>>
>> This looks wrong to me.  Requests can complete in any order, can they
>> not?  So if request 0 did not complete and request 1 did not,
>> you send avail - inuse and on the secondary you will process and
>> complete request 1 the second time, crashing the guest.
>
> In case of Kemari, no.  We sit between devices and net/block, and
> queue the requests.  After completing each transaction, we flush
> the requests one by one.  So there won't be completion inversion,
> and therefore won't be visible to the guest.
>
> Yoshi
>
>>
>>>
>>> >
>>> >> >
>>> >> >> ---
>>> >> >>  hw/virtio.c |    8 +++++++-
>>> >> >>  1 files changed, 7 insertions(+), 1 deletions(-)
>>> >> >>
>>> >> >> diff --git a/hw/virtio.c b/hw/virtio.c
>>> >> >> index 849a60f..5509644 100644
>>> >> >> --- a/hw/virtio.c
>>> >> >> +++ b/hw/virtio.c
>>> >> >> @@ -72,7 +72,7 @@ struct VirtQueue
>>> >> >>      VRing vring;
>>> >> >>      target_phys_addr_t pa;
>>> >> >>      uint16_t last_avail_idx;
>>> >> >> -    int inuse;
>>> >> >> +    uint16_t inuse;
>>> >> >>      uint16_t vector;
>>> >> >>      void (*handle_output)(VirtIODevice *vdev, VirtQueue *vq);
>>> >> >>      VirtIODevice *vdev;
>>> >> >> @@ -671,6 +671,7 @@ void virtio_save(VirtIODevice *vdev, QEMUFile *f)
>>> >> >>          qemu_put_be32(f, vdev->vq[i].vring.num);
>>> >> >>          qemu_put_be64(f, vdev->vq[i].pa);
>>> >> >>          qemu_put_be16s(f, &vdev->vq[i].last_avail_idx);
>>> >> >> +        qemu_put_be16s(f, &vdev->vq[i].inuse);
>>> >> >>          if (vdev->binding->save_queue)
>>> >> >>              vdev->binding->save_queue(vdev->binding_opaque, i, f);
>>> >> >>      }
>>> >> >> @@ -711,6 +712,11 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f)
>>> >> >>          vdev->vq[i].vring.num = qemu_get_be32(f);
>>> >> >>          vdev->vq[i].pa = qemu_get_be64(f);
>>> >> >>          qemu_get_be16s(f, &vdev->vq[i].last_avail_idx);
>>> >> >> +        qemu_get_be16s(f, &vdev->vq[i].inuse);
>>> >> >> +
>>> >> >> +        /* revert last_avail_idx if there are outstanding emulation. */
>>> >> >> +        vdev->vq[i].last_avail_idx -= vdev->vq[i].inuse;
>>> >> >> +        vdev->vq[i].inuse = 0;
>>> >> >>
>>> >> >>          if (vdev->vq[i].pa) {
>>> >> >>              virtqueue_init(&vdev->vq[i]);
>>> >> >> --
>>> >> >> 1.7.1.2
>>> >> >>
>>> >> >> --
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