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Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers a
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed |
Date: |
Wed, 12 Dec 2012 16:52:53 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0 |
Il 12/12/2012 16:25, Michael S. Tsirkin ha scritto:
> On Wed, Dec 12, 2012 at 04:01:27PM +0100, Paolo Bonzini wrote:
>> Il 12/12/2012 15:47, Michael S. Tsirkin ha scritto:
>>>> Ok, so we need some API for virtio-{blk,scsi} to communicate back the
>>>> indexes of in-flight requests to virtio. The indexes are known from the
>>>> VirtQueueElement, so that's fine.
>>>>
>>>> Even better would be a virtio_save_request/virtio_load_request API...
>>>
>>> So you are saying this is a bug then? Great.
>>
>> I'm not sure what you mean by "it will never put the missing heads
>> in the used ring". The serialized requests are put in the used rings
>> when they are completed and virtio-{blk,scsi} calls virtqueue_push. Is
>> the problem if you have a vring that looks like this:
>>
>> A A U A U U A A
>>
>> ? Which heads are leaked? {0,1}, {2} or {6,7}? Or a combination thereof?
>
> I don't know what A A U A U U A A means.
Right, not very clear.... It means that descriptor 2 and 4 and 5 are
free, while the others are in-flight.
> To make it work, complete all requests when vm is stopped.
That's not a choice, sorry.
>>> This is exactly what the assert above is out there to catch.
>>> And you really can't fix it without breaking migration compatibility.
>>
>> Why not? The index in the vring is in the migration data.
>
> index is not enough if requests are outstanding.
Sorry, I meant the descriptor index.
> Pls check the example in the log of the patch.
I'm likewise not sure what you meant by
A 1
A 2
U 2
A 2
U 2
A 2
U 2
A 2 <---
U 2
If I understand it, before the point marked with the arrow, the avail
ring is
1 2 2 2
vring_avail_idx(vq) == 3
last_avail_idx == 3
After, it is
2 2 2 2
vring_avail_idx(vq) == 4
last_avail_idx == 3
What's wrong with that?
You wrote "the only way to know head 1 is outstanding is because backend
has stored this info somewhere". But the backend _is_ tracking it (by
serializing and then restoring the VirtQueueElement) and no leak happens
because virtqueue_fill/flush will put the head on the used ring sooner
or later.
>>> As step 1, I think we should just complete all outstanding
>>> requests when VM stops.
>>>
>>> Yes it means you can't do the retry hack after migration
>>> but this is hardly common scenario.
>>
>> I disagree...
>
> Disagree with what? You are saying it's common?
It's not common, but you cannot block migration because you have an I/O
error. Solving the error may involve migrating the guests away from
that host.
Paolo
- [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Michael S. Tsirkin, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Stefan Hajnoczi, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Paolo Bonzini, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Michael S. Tsirkin, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Paolo Bonzini, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Michael S. Tsirkin, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Paolo Bonzini, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Michael S. Tsirkin, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed,
Paolo Bonzini <=
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Michael S. Tsirkin, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Paolo Bonzini, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Michael S. Tsirkin, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Paolo Bonzini, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Michael S. Tsirkin, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Paolo Bonzini, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Michael S. Tsirkin, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Anthony Liguori, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Michael S. Tsirkin, 2012/12/12
- Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed, Rusty Russell, 2012/12/13