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 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?

Prior to the first one, I think. It should not change again, should it?

>
>> 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.

If we migrate in from an older QEMU, we don't know whether we are
dealing with legacy or not, until feature negotiation is already
done... don't we have to ask the transport?