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Re: [Qemu-devel] [PATCH 0/2] virtio len fixes for qemu.

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH 0/2] virtio len fixes for qemu.
Date: Wed, 18 Mar 2015 13:32:44 +0100

On Wed, Mar 11, 2015 at 04:29:30PM +1030, Rusty Russell wrote:
> The virtio 'used' ring describes descriptors which have been used.  It
> also says how many bytes have been written to the ring.  For some cases,
> this value is ignored by Linux guests, thus errors have not been noticed.
> I was working on increasing the checking in Linux when I noticed this
> behaviour.
> The first patch changes the 'len' formal parameter name to 'len_written' to
> make the API clearer, and adds an assert(). The second fixes block writes.
> Cheers,
> Rusty.
> PS.  It's based on MST's virtio-1.0 tree, but should be easily ported.

After going back and forth on this, I decided it's
best to defer this change to 2.4.
Guests can't depend on this behaviour without checking virtio-1 anyway.

> Rusty Russell (2):
>   virtio: make it clear that "len" for a used descriptor is len written.
>   virtio-blk: fix length calculations for write operations.
>  hw/block/virtio-blk.c      |  9 ++++++++-
>  hw/virtio/virtio.c         | 19 ++++++++++++-------
>  include/hw/virtio/virtio.h |  4 ++--
>  3 files changed, 22 insertions(+), 10 deletions(-)
> -- 
> 2.1.0

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