linux-kernel - Re: [PATCH] virtio-balloon: Document byte ordering of poison

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Message-ID: <20200714045454-mutt-send-email-mst@kernel.org>
Date:   Tue, 14 Jul 2020 04:55:02 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     david@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtio-dev@...ts.oasis-open.org
Subject: Re: [PATCH] virtio-balloon: Document byte ordering of poison_val

On Mon, Jul 13, 2020 at 01:35:39PM -0700, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> 
> The poison_val field in the virtio_balloon_config is treated as a
> little-endian field by the host. Since we are currently only having to deal
> with a single byte poison value this isn't a problem, however if the value
> should ever expand it would cause byte ordering issues. Document that in
> the code so that we know that if the value should ever expand we need to
> byte swap the value on big-endian architectures.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...ux.intel.com>

Queued, thanks!

> ---
>  drivers/virtio/virtio_balloon.c |    5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 1f157d2f4952..d0fd8f8dc6ed 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -974,6 +974,11 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  		/*
>  		 * Let the hypervisor know that we are expecting a
>  		 * specific value to be written back in balloon pages.
> +		 *
> +		 * If the PAGE_POISON value was larger than a byte we would
> +		 * need to byte swap poison_val here to guarantee it is
> +		 * little-endian. However for now it is a single byte so we
> +		 * can pass it as-is.
>  		 */
>  		if (!want_init_on_free())
>  			memset(&poison_val, PAGE_POISON, sizeof(poison_val));