linux-kernel - Re: [RFC PATCH 1/1] virtio: write back features before verify

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Date:   Fri, 1 Oct 2021 16:34:34 +0200
From:   Christian Borntraeger <borntraeger@...ibm.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:     markver@...ibm.com, Cornelia Huck <cohuck@...hat.com>,
        linux-s390@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] virtio: write back features before verify

Am 30.09.21 um 03:20 schrieb Halil Pasic:
> This patch fixes a regression introduced by commit 82e89ea077b9
> ("virtio-blk: Add validation for block size in config space") and
> enables similar checks in verify() on big endian platforms.
> 
> The problem with checking multi-byte config fields in the verify
> callback, on big endian platforms, and with a possibly transitional
> device is the following. The verify() callback is called between
> config->get_features() and virtio_finalize_features(). That we have a
> device that offered F_VERSION_1 then we have the following options
> either the device is transitional, and then it has to present the legacy
> interface, i.e. a big endian config space until F_VERSION_1 is
> negotiated, or we have a non-transitional device, which makes
> F_VERSION_1 mandatory, and only implements the non-legacy interface and
> thus presents a little endian config space. Because at this point we
> can't know if the device is transitional or non-transitional, we can't
> know do we need to byte swap or not.
> 
> The virtio spec explicitly states that the driver MAY read config
> between reading and writing the features so saying that first accessing
> the config before feature negotiation is done is not an option. The
> specification ain't clear about setting the features multiple times
> before FEATURES_OK, so I guess that should be fine.
> 
> I don't consider this patch super clean, but frankly I don't think we
> have a ton of options. Another option that may or man not be cleaner,
> but is also IMHO much uglier is to figure out whether the device is
> transitional by rejecting _F_VERSION_1, then resetting it and proceeding
> according tho what we have figured out, hoping that the characteristics
> of the device didn't change.
> 
> Signed-off-by: Halil Pasic <pasic@...ux.ibm.com>
> Fixes: 82e89ea077b9 ("virtio-blk: Add validation for block size in config space")
> Reported-by: markver@...ibm.com

Just to make this more obvious. Since 5.14 DASD devices as backing for virtio-blk no
longer work as the block size is no longer reported to the guest. So we need a fix
for the issue.