Re: Qemu and virtio 1.0

Cornelia Huck
2015-02-25T11:26:31Z

On Wed, 25 Feb 2015 14:50:22 +1030
Rusty Russell <rusty@rustcorp.com.au> wrote:

> OK, I am trying to experiment with virtio 1.0 support using the
> latest kernel and MST's qemu tree:
>
> https://git.kernel.org/cgit/virt/kvm/mst/qemu.git/?h=virtio-1.0
>
> The first issue is that the device config endian was wrong (see
> attached patch).
>
> I'm now setting up a BE guest on my x86 laptop, and a BE and LE guest
> on a BE powerpc machine, to check that all combinations work correctly.
> If others test too, that would be appreciated!

My virtio-1 work had been taking the back seat recently, but I plan to
look into it again soon.

>
> Cheers,
> Rusty.
>
> From 95ac91554ed602f856a2a5fcc25eaffcad1b1c8d Mon Sep 17 00:00:00 2001
> From: Rusty Russell <rusty@rustcorp.com.au>
> Date: Tue, 24 Feb 2015 14:47:44 +1030
> Subject: [PATCH] virtio_config_write*/virtio_config_read*: Don't endian swap
> for virtio 1.0.

Ah, virtio-ccw doesn't use these config space accessors, that's why I
did not look at them.

>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 079944c..882a31b 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -662,7 +662,12 @@ uint32_t virtio_config_readw(VirtIODevice *vdev, uint32_t addr)
>
> k->get_config(vdev, vdev->config);
>
> - val = lduw_p(vdev->config + addr);
> + /* Virtio 1.0 is always LE */
> + if (virtio_has_feature(vdev, VIRTIO_F_VERSION_1)) {
> + val = lduw_le_p(vdev->config + addr);
> + } else {
> + val = lduw_p(vdev->config + addr);
> + }

Can't you use the virtio_* helpers for that?

> return val;
> }
>

<looks at the callers>

It seems virtio-pci does some conditional swapping based on
virtio-endianness already, which should account for virtio-1.
virtio-mmio does not seem to do so.

But:

Device code accessing config space already does use the virtio
accessors - or virtio-ccw would not work as it simply copies the whole
config space. So it seems this change simply undos the endian swap in
virtio-pci (while probably breaking virtio-mmio). Can we simply get rid
of the endian swap in virtio-pci instead, or have I been throuroughly
confused?