Cornelia Huck
2021-Oct-04 14:33 UTC
[RFC PATCH 1/1] virtio: write back features before verify
On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst at 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 at 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 at 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...
Michael S. Tsirkin
2021-Oct-04 15:07 UTC
[RFC PATCH 1/1] virtio: write back features before verify
On Mon, Oct 04, 2021 at 04:33:21PM +0200, Cornelia Huck wrote:> On Mon, Oct 04 2021, "Michael S. Tsirkin" <mst at 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 at 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 at 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