drm/virtio: sync shmem backing on guest-bound transfers

virtio_gpu_cmd_transfer_to_host_{2d,3d}() sync the shmem backing for the
device before the transfer, but nothing syncs for the CPU when a transfer
runs the other way. That breaks two ways. Where the DMA layer bounces, the
device writes into the bounce buffer while the guest keeps reading the
original pages. Where DMA is not coherent, the device writes memory while
the CPU keeps stale cache lines, because nothing reaches
arch_sync_dma_for_cpu(). Either way DRM_IOCTL_VIRTGPU_TRANSFER_FROM_HOST
hands back stale data.

Sashiko originally found this in
https://lore.kernel.org/dri-devel/20260806231002.27B4D1F000E9@smtp.kernel.org
but the suggestion there to fix this with dma_sync_sgtable_for_cpu()
isn't a sufficient fix, for two reasons.

- The transfer is asynchronous. virtio_gpu_cmd_transfer_from_host_3d() only
queues the command, so a sync there would run before the device had written
anything. It belongs on completion, and ahead of any fence signalling.
A waiter woken by the fence would otherwise race the sync and read the
backing pages regardless. It needs its own pass over the reclaim list
rather than a step inside the existing one, because
virtio_gpu_fence_event_process() also signals every earlier fence in the
same context, so any entry in that loop may signal an earlier entry's
fence.

- The transfer is also partial, carrying an offset, a level and a box.
Where the mapping bounces, a sync for the CPU copies the whole mapping
back, so unless the mapping is primed first the regions the device did not
write come back holding whatever the bounce buffer contained, discarding
data the guest owned.

So the fix: Prime the mapping before queueing, tag the vbuffer, and sync
for the CPU on completion before the fence is signalled.

A second transfer must not snapshot the mapping while an earlier one is
still in flight, or the snapshot would predate whatever the CPU wrote once
the earlier fence signalled and the later sync would discard it.

To mitigate this, wait for outstanding fences under the reservation before
priming.

Neither sync copies anything unless the mapping genuinely bounces:
swiotlb_sync_single_for_cpu() and its Xen counterpart look the address up
in the bounce pool and return early when it is absent. On a platform with
non-coherent DMA they still perform the necessary cache maintenance.

The range cannot be narrowed to the box, since for a non-blob resource
virtio_gpu_transfer_from_host_ioctl() rejects a caller-supplied stride and
layer_stride, leaving the layout to the host and the guest with no way to
work out which bytes the device writes. A host3d guest blob does carry
both, so its extent could be bounded, but the sync is left whole there too
rather than special-cased: priming makes the untouched regions round-trip
unchanged either way.

Behaviour changes worth noting:

- TRANSFER_FROM_HOST can now block, where before it returned as soon as the
command was queued. Repeated readbacks of one resource serialise, and a
readback can wait behind an earlier queued command that touched it, since
virtio_gpu_array_add_fence() tags uploads, execbufs and plane flushes
alike with DMA_RESV_USAGE_WRITE. -ERESTARTSYS was already possible here
via dma_resv_lock_interruptible().

- A CPU write racing an in-flight transfer to the same resource is now
lost, where before it survived and the transfer was lost instead. Priming
captures the pages as of queueing, so a write landing before completion is
overwritten by the sync.

- TRANSFER_TO_HOST can also block now, but only while a guest-bound
transfer on the same resource is outstanding, which happens only for
callers that issue both without waiting.

- Where a batch of completions contains a guest-bound transfer, the sync
pass delays fence signalling for the whole batch. Only bounced pages are
copied and the swiotlb pool bounds it. A batch with no such transfer is
unaffected.

Tested under QEMU on x86 with swiotlb=force and virtio-vga-gl
iommu_platform=on, which forces both preconditions required to hit the
original bug.

Fixes: a3b815f09bb8 ("drm/virtio: add iommu support.")
Reported-by: Sashiko AI review <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/dri-devel/20260806231002.27B4D1F000E9@smtp.kernel.org/
Signed-off-by: Benjamin Leggett <benjamin@edera.io>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patch.msgid.link/20260814-virtgpu-from-host-sync-v4-1-64dd736b1779@edera.io

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