Hi Akihiko,
> Subject: Re: [PATCH v1 3/7] virtio-gpu: Find hva for Guest's DMA addr
> associated with a ram device
>
> On 2025/10/04 8:35, 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]>
> > Signed-off-by: Vivek Kasireddy <[email protected]>
> > ---
> > hw/display/virtio-gpu.c | 30 +++++++++++++++++++++++++++---
> > 1 file changed, 27 insertions(+), 3 deletions(-)
> >
> > diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
> > index df696611a4..22bbe76809 100644
> > --- a/hw/display/virtio-gpu.c
> > +++ b/hw/display/virtio-gpu.c
> > @@ -801,6 +801,32 @@ 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;
> > +
> > + if (cmd->cmd_hdr.type != VIRTIO_GPU_CMD_RESOURCE_CREATE_BLOB)
> {
>
> I wonder what this conditional for. I think this breaks mapping VFIO
> with VIRTIO_GPU_CMD_RESOURCE_ATTACH_BACKING.
You are right, it does break attach_backing cmd. I'll remove it.
Thanks,
Vivek
>
> > + return dma_memory_map(VIRTIO_DEVICE(g)->dma_as, a, len,
> > + DMA_DIRECTION_TO_DEVICE,
> > + MEMTXATTRS_UNSPECIFIED);
> > + }
> > +
> > + 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);
> > +}
> > +
> > int virtio_gpu_create_mapping_iov(VirtIOGPU *g,
> > uint32_t nr_entries, uint32_t offset,
> > struct virtio_gpu_ctrl_command *cmd,
> > @@ -842,9 +868,7 @@ 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);
