Hi Akihiko,
> Subject: Re: [PATCH v4 2/8] virtio-gpu: Find hva for Guest's DMA addr
> associated with a ram device
>
> On 2026/01/23 15:17, Vivek Kasireddy wrote:
> > If the Guest provides a DMA address that is associated with a ram
> > device (such as a 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().
> >
> > This is because the ram device's address space is not accessible
> > to virtio-gpu directly and hence dma_memory_map() cannot be used.
>
> This description is a bit confusing.
I agree; the description does seem ambiguous.
>
> The first problem is with the phrase "the ram device's address space".
> The address space we are dealing with is always
> VIRTIO_DEVICE(g)->dma_as. More precisely it is a memory region, not
> address space.
>
> The second problem is that it says the ram device's memory region is not
> directly accessible. It is usually true, but here we are directly
> accessing it anyway because the guest knows how to do so.
Does this look better:
"virtio-gpu: Find hva for Guest's DMA addr associated with a ram device
If the Guest provides a DMA address that is associated with a ram
device (such as a 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() in this case to get the hva because
it can only be used with addresses that are associated with the
system (or physical) memory region. Therefore, in order to handle
addresses associated with ram 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."
Thanks,
Vivek
>
> Regards,
> Akihiko Odaki
>
> > Therefore, we first need to identify the memory region associated
> > with the DMA address and add the offset to the pointer returned
> > from memory_region_get_ram_ptr() to obtain the host address.
> >
> > Note that we take a reference on the memory region but we would
> > still eventually call dma_memory_unmap() (to unref the mr).
> >
> > 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]>
> > 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 3d58cebf4b..9930a21317 100644
> > --- a/hw/display/virtio-gpu.c
> > +++ b/hw/display/virtio-gpu.c
> > @@ -798,6 +798,23 @@ 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;
> > + }
> > + return NULL;
> > +}
> > +
> > int virtio_gpu_create_mapping_iov(VirtIOGPU *g,
> > uint32_t nr_entries, uint32_t offset,
> > struct virtio_gpu_ctrl_command *cmd,
> > @@ -839,9 +856,7 @@ int virtio_gpu_create_mapping_iov(VirtIOGPU
> *g,
> >
> > 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);
