Hi Akihiko,
> Subject: Re: [PATCH v2 02/10] virtio-gpu: Find hva for Guest's DMA addr
> associated with a ram device
>
> On 2025/11/09 14:33, 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.
> > Therefore, we first need to identify the memory region associated with
> > the DMA address and figure out if it belongs to a ram device or not
> > and decide how to obtain the host address accordingly.
> >
> > Note that we take a reference on the memory region if it belongs to a
> > ram device but we would still call dma_memory_unmap() later (to unref
> > mr) regardless of how we obtained the hva.
> >
> > 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 | 24 +++++++++++++++++++++---
> > 1 file changed, 21 insertions(+), 3 deletions(-)
> >
> > diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c index
> > 199b18c746..d352b5afd6 100644
> > --- a/hw/display/virtio-gpu.c
> > +++ b/hw/display/virtio-gpu.c
> > @@ -798,6 +798,26 @@ 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 = 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);
>
> This function should:
> - call memory_region_get_ram_ptr(mr)
> if memory_region_is_ram(mr)
> - return NULL otherwise
>
> There are a few reasons. First, the documentation of dma_memory_map()
> tells to use it "only for reads OR writes - not for read-modify-write
> operations." It can be used for read-modify-write operations so
> dma_memory_map() should be avoided.
This patch series only deals with non-virgl use-cases where AFAICS resources
are not written to on the Host.
>
> Second, it ensures that the mapped pointer is writable.
> "[PATCH v2 10/10] virtio-gpu-dmabuf: Create dmabuf for blobs associated
> with VFIO devices" adds checks for memory_region_is_ram() and
> memory_region_is_ram_device() to virtio_gpu_init_dmabuf(), but the other
> callers also use the function to map writable pointers.
Unless I am missing something, I don't see where writable pointers are used
in non-virgl use-cases?
>
> It also makes the check of memory_region_is_ram_device() and
> memory_region_is_ram() unnecessary for virtio_gpu_init_dmabuf(), reducing
> the overall complexity.
Since buffers reside completely in either ram or ram_device regions, using both
memory_region_is_ram_device() and memory_region_is_ram() to check where
they are located seems necessary and unavoidable.
Thanks,
Vivek
>
> Regards,
> Akihiko Odaki
>
> > +}
> > +
> > int virtio_gpu_create_mapping_iov(VirtIOGPU *g,
> > uint32_t nr_entries, uint32_t offset,
> > struct virtio_gpu_ctrl_command
> > *cmd, @@ -839,9 +859,7 @@ 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);
