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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 04, 2021 at 04:33:21PM +0200, Cornelia Huck wrote:
> 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.

Basically on each device config access?

> 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...

It's just a shorthand to minimize changes. No need to migrate I think.

-- 
MST