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Re: [Qemu-devel] [PATCH v2 0/2] migration: fix virtio-rng

From: Amit Shah
Subject: Re: [Qemu-devel] [PATCH v2 0/2] migration: fix virtio-rng
Date: Mon, 17 Apr 2017 21:33:51 +0200

On (Wed) 12 Apr 2017 [15:53:10], Laurent Vivier wrote:
> When post-copy migration is enabled, the destination
> guest can ask for memory from the source when the
> vmstate is restored.
> In the case of virtio, a part of the virtqueue
> is migrated by the vmstate structure (last_avail_idx)
> another part is migrated inside the RAM (used_idx).
> On the source side, the virtqueue can be modified
> whereas the vmstate is already migrated, and the destination
> side can ask for the value in RAM. In this case we have
> an inconsistency that can generate this kind of error:
>     "VQ 0 size 0x8 < last_avail_idx 0xa - used_idx 0"
> in hw/virtio/virtio.c:2180, virtio_load().
> This happens with virtio-rng as the chr_read()
> function which modifies the virqueue is called
> by the rng backend and the rng backend continues to
> run while the migration is running and the CPU is stopped.
> This series fixes this problem by ignoring chr_read()
> calls while the CPU is stopped. The first patch of the
> series fixes another problem triggered by this error
> case: a use-after-free case.
> The probability to have this problem is very low, as
> generally the post-copy phase is very short, so the window
> to modify the virtqueue while the vmstate has been sent
> is very small... except if you are doing trans-continental
> guest migration with high latency and post-copy phase that
> can be run for minutes.
> I've been able to reproduce the problem locally on a host,
> by adding network latency with "tc". Another condition is
> to have an rng daemon running in the guest to generate
> events in the virtio-rng device.

Acked-by: Amit Shah <address@hidden>


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