linux-kernel - Re: [PATCH 1/1] virtio: write back F_VERSION

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Date:   Thu, 07 Oct 2021 13:52:24 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Halil Pasic <pasic@...ux.ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Xie Yongji <xieyongji@...edance.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Cc:     Halil Pasic <pasic@...ux.ibm.com>, markver@...ibm.com,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-s390@...r.kernel.org, stefanha@...hat.com,
        Raphael Norwitz <raphael.norwitz@...anix.com>,
        qemu-devel@...gnu.org
Subject: Re: [PATCH 1/1] virtio: write back F_VERSION_1 before validate

On Wed, Oct 06 2021, Halil Pasic <pasic@...ux.ibm.com> wrote:

> The virtio specification virtio-v1.1-cs01 states: Transitional devices
> MUST detect Legacy drivers by detecting that VIRTIO_F_VERSION_1 has not
> been acknowledged by the driver.  This is exactly what QEMU as of 6.1
> has done relying solely on VIRTIO_F_VERSION_1 for detecting that.
>
> However, the specification also says: driver MAY read (but MUST NOT
> write) the device-specific configuration fields to check that it can
> support the device before setting FEATURES_OK.

Suggest to put the citations from the spec into quotes, so that they are
distinguishable from the rest of the text.

>
> In that case, any transitional device relying solely on
> VIRTIO_F_VERSION_1 for detecting legacy drivers will return data in
> legacy format.  In particular, this implies that it is in big endian
> format for big endian guests. This naturally confuses the driver which
> expects little endian in the modern mode.
>
> It is probably a good idea to amend the spec to clarify that
> VIRTIO_F_VERSION_1 can only be relied on after the feature negotiation
> is complete. However, we already have regression so let's try to address

s/regression/a regression/

> it.

Maybe mention what the regression is?

Also mention that we use this workaround for modern on BE only?

>
> Signed-off-by: Halil Pasic <pasic@...ux.ibm.com>
> Fixes: 82e89ea077b9 ("virtio-blk: Add validation for block size in config space")
> Fixes: fe36cbe0671e ("virtio_net: clear MTU when out of range")
> Reported-by: markver@...ibm.com
> ---
>  drivers/virtio/virtio.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 0a5b54034d4b..494cfecd3376 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -239,6 +239,16 @@ static int virtio_dev_probe(struct device *_d)
>  		driver_features_legacy = driver_features;
>  	}
>  
> +	/*
> +	 * Some devices detect legacy solely via F_VERSION_1. Write
> +	 * F_VERSION_1 to force LE for these when needed.

"...to force LE config space accesses before FEATURES_OK for these when
needed (BE)."

?

> +	 */
> +	if (drv->validate && !virtio_legacy_is_little_endian()
> +			  && BIT_ULL(VIRTIO_F_VERSION_1) & device_features) {

Nit: putting device_features first would read more naturally to me.

> +		dev->features = BIT_ULL(VIRTIO_F_VERSION_1);
> +		dev->config->finalize_features(dev);
> +	}
> +
>  	if (device_features & (1ULL << VIRTIO_F_VERSION_1))
>  		dev->features = driver_features & device_features;
>  	else

Patch LGTM.