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

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Oct 04, 2021 at 02:19:55PM +0200, Cornelia Huck wrote:
>> 
>> [cc:qemu-devel]
>> 
>> On Sat, Oct 02 2021, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> 
>> > On Fri, Oct 01, 2021 at 09:21:25AM +0200, Halil Pasic wrote:
>> >> On Thu, 30 Sep 2021 07:12:21 -0400
>> >> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> >> 
>> >> > On Thu, Sep 30, 2021 at 03:20:49AM +0200, Halil Pasic wrote:
>> >> > > 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.  
>> >> > 
>> >> > Hmm which transport does this refer to?
>> >> 
>> >> It is the same with virtio-ccw and virtio-pci. I see the same problem
>> >> with both on s390x. I didn't try with virtio-blk-pci-non-transitional
>> >> yet (have to figure out how to do that with libvirt) for pci I used
>> >> virtio-blk-pci.
>> >> 
>> >> > Distinguishing between legacy and modern drivers is transport
>> >> > specific.  PCI presents
>> >> > legacy and modern at separate addresses so distinguishing
>> >> > between these two should be no trouble.
>> >> 
>> >> You mean the device id? Yes that is bolted down in the spec, but
>> >> currently we don't exploit that information. Furthermore there
>> >> is a fat chance that with QEMU even the allegedly non-transitional
>> >> devices only present a little endian config space after VERSION_1
>> >> was negotiated. Namely get_config for virtio-blk is implemented in
>> >> virtio_blk_update_config() which does virtio_stl_p(vdev,
>> >> &blkcfg.blk_size, blk_size) and in there we don't care
>> >> about transitional or not:
>> >> 
>> >> static inline bool virtio_access_is_big_endian(VirtIODevice *vdev)
>> >> {
>> >> #if defined(LEGACY_VIRTIO_IS_BIENDIAN)
>> >>     return virtio_is_big_endian(vdev);
>> >> #elif defined(TARGET_WORDS_BIGENDIAN)
>> >>     if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
>> >>         /* Devices conforming to VIRTIO 1.0 or later are always LE. */
>> >>         return false;
>> >>     }
>> >>     return true;
>> >> #else
>> >>     return false;
>> >> #endif
>> >> }
>> >> 
>> >
>> > ok so that's a QEMU bug. Any virtio 1.0 and up
>> > compatible device must use LE.
>> > It can also present a legacy config space where the
>> > endian depends on the guest.
>> 
>> So, how is the virtio core supposed to determine this? A
>> transport-specific callback?
>
> I'd say a field in VirtIODevice is easiest.

The transport needs to set this as soon as it has figured out whether
we're using legacy or not. I guess we also need to fence off any
accesses respectively error out the device if the driver tries any
read/write operations that would depend on that knowledge?

And using a field in VirtIODevice would probably need some care when
migrating. Hm...