On 2026/10/09 13:32, Kasireddy, Vivek wrote:
Hi Akihiko,
Subject: Re: [PATCH v13 1/9] virtio-gpu: Find hva for Guest's DMA addr
associated with a ram device
On 2026/10/08 5:09, Vivek Kasireddy wrote:
If the Guest provides a DMA address that is associated with a ram
device (such as a VFIO PCI device region and not its system memory),
then we can obtain the hva (host virtual address) by invoking
address_space_translate() followed by memory_region_get_ram_ptr().
We cannot use dma_memory_map() because for memory regions that
do
not support direct access, it would create bounce buffers instead
of returning the actual hva, which is not desirable here. And, since
VFIO device regions are not considered directly accessible (because
they are mostly MMIO based), virtio-gpu cannot invoke
dma_memory_map()
to obtain the hva in this case.
Therefore, in order to handle addresses associated with VFIO devices,
we need to use the address_space_translate() API to first identify
the right memory region and the appropriate offset within that
region and then use memory_region_get_ram_ptr() to get the hva.
This approach also works for addresses associated with the system
memory region.
Note that, although we take an explicit reference on the memory
region, we would still rely on dma_memory_unmap() to drop that
reference when the dma mapping is eventually unmapped.
Cc: Marc-André Lureau <[email protected]>
Cc: Alex Bennée <[email protected]>
Cc: Akihiko Odaki <[email protected]>
Cc: Dmitry Osipenko <[email protected]>
Cc: Alex Williamson <[email protected]>
Cc: Cédric Le Goater <[email protected]>
Reviewed-by: Akihiko Odaki <[email protected]>
Signed-off-by: Vivek Kasireddy <[email protected]>
---
hw/display/virtio-gpu.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
index bb0f41bb1e..386db05e5b 100644
--- a/hw/display/virtio-gpu.c
+++ b/hw/display/virtio-gpu.c
@@ -948,6 +948,23 @@ static void
virtio_gpu_set_scanout_blob(VirtIOGPU *g,
&fb, res, &ss.r, &cmd->error);
}
+static void *virtio_gpu_dma_memory_map(VirtIOGPU *g,
+ struct virtio_gpu_ctrl_command *cmd,
+ uint64_t a, hwaddr *len)
+{
+ MemoryRegion *mr;
+ hwaddr xlat;
+
+ mr = address_space_translate(VIRTIO_DEVICE(g)->dma_as, a,
&xlat, len,
+ DMA_DIRECTION_TO_DEVICE,
+ MEMTXATTRS_UNSPECIFIED);
+ if (memory_region_is_ram(mr)) {
+ memory_region_ref(mr);
+ return memory_region_get_ram_ptr(mr) + xlat;
On rechecking this, I don't think it is safe with Xen.
memory_region_get_ram_ptr(mr) + xlat assumes a contiguous host
mapping
of the entire region. Xen maps guest RAM on demand through its
mapcache.
Here, memory_region_get_ram_ptr() obtains a mapping at offset zero
without pinning the cache entry, and adding xlat afterward does not
ensure that the requested range is mapped.
The MemoryRegion reference keeps the region alive, but does not
prevent
the mapcache entry from being reused. Resource backing can therefore
receive an invalid or unpinned pointer.
I think we need to extend dma_memory_map() for this use case instead
of
replacing it, preserving its mapping and unmapping semantics.
I am not sure if extending dma_memory_map() might be a better/cleaner
solution for this use-case. If obtaining the hva/host ptr via
memory_region_get_ram_ptr(mr) + xlat for guest RAM is not safe with
Xen, then would it not make more sense to go back to using dma_memory_map()
for Guest RAM and memory_region_get_ram_ptr(mr) + xlat for VFIO
backed regions? That is, doing the following:
static void *virtio_gpu_dma_memory_map(VirtIOGPU *g,
struct virtio_gpu_ctrl_command *cmd,
uint64_t a, hwaddr *len)
{
MemoryRegion *mr = NULL;
hwaddr xlat;
mr = address_space_translate(VIRTIO_DEVICE(g)->dma_as, a, &xlat, len,
DMA_DIRECTION_TO_DEVICE,
MEMTXATTRS_UNSPECIFIED);
if (memory_region_is_ram_device(mr)) {
memory_region_ref(mr);
return memory_region_get_ram_ptr(mr) + xlat;
}
return dma_memory_map(VIRTIO_DEVICE(g)->dma_as, a, len,
DMA_DIRECTION_TO_DEVICE,
MEMTXATTRS_UNSPECIFIED);
}
Checking this logic again, I found this patch is not necessary since
commit ff77a515113f ("system/memory: Make ram device region directly
accessible"). We can just drop it.
Regards,
Akihiko Odaki
Thanks,
Vivek
Regards,
Akihiko Odaki
+ }
+ return NULL;
+}
+
int virtio_gpu_create_mapping_iov(VirtIOGPU *g,
uint32_t nr_entries, uint32_t offset,
struct virtio_gpu_ctrl_command *cmd,
@@ -993,9 +1010,7 @@ int virtio_gpu_create_mapping_iov(VirtIOGPU
*g,
/* TODO: a common DMA map SG helper */
do {
len = l;
- map = dma_memory_map(VIRTIO_DEVICE(g)->dma_as, a,
&len,
- DMA_DIRECTION_TO_DEVICE,
- MEMTXATTRS_UNSPECIFIED);
+ map = virtio_gpu_dma_memory_map(g, cmd, a, &len);
if (!map) {
qemu_log_mask(LOG_GUEST_ERROR, "%s: failed to map
MMIO memory for"
" element %d\n", __func__, e);
