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virtio/gpu: mmap dma-buf blob fds into the guest shm region #644

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Merged
slp merged 1 commit into from
Apr 22, 2026
Merged

virtio/gpu: mmap dma-buf blob fds into the guest shm region #644

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22 changes: 20 additions & 2 deletions src/devices/src/virtio/gpu/virtio_gpu.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,8 @@ use krun_display::{
use libc::c_void;
#[cfg(target_os = "macos")]
use rutabaga_gfx::RUTABAGA_MEM_HANDLE_TYPE_APPLE;
#[cfg(all(feature = "virgl_resource_map2", target_os = "linux"))]
use rutabaga_gfx::RUTABAGA_MEM_HANDLE_TYPE_DMABUF;
#[cfg(all(not(feature = "virgl_resource_map2"), target_os = "linux"))]
use rutabaga_gfx::RUTABAGA_MEM_HANDLE_TYPE_OPAQUE_FD;
#[cfg(all(feature = "virgl_resource_map2", target_os = "linux"))]
Expand Down Expand Up @@ -837,7 +839,20 @@ impl VirtioGpu {
let addr = shm_region.host_addr + offset;

if let Ok(export) = self.rutabaga.export_blob(resource_id) {
if export.handle_type == RUTABAGA_MEM_HANDLE_TYPE_SHM {
// SHM and DMABUF are both regular host fds whose pages can be exposed
// to the guest by mmap'ing them directly into the virtio shm region.
// For SHM (memfd) this has always worked. For DMABUF it had been
// delegated to virgl_renderer_resource_map2, which only handles
// virglrenderer-allocated GPU memory and silently no-ops for external
// dma-bufs — leaving the guest blob backed by zero pages. That broke
// muvm camera capture, where the v4l2 source exports kernel buffers
// via VIDIOC_EXPBUF as dma-bufs, the muvm bridge forwards the fd
// across SCM_RIGHTS, libkrun classifies it as DMABUF, and the guest's
// CREATE_BLOB allocates a host-backed-by-nothing blob. Mapping the
// dma-buf fd directly here gives the guest real, live pages.
if export.handle_type == RUTABAGA_MEM_HANDLE_TYPE_SHM
|| export.handle_type == RUTABAGA_MEM_HANDLE_TYPE_DMABUF
{
let ret = unsafe {
libc::mmap(
addr as *mut libc::c_void,
Expand All @@ -849,7 +864,10 @@ impl VirtioGpu {
)
};
if ret == libc::MAP_FAILED {
error!("failed to mmap resource in shm region");
error!(
"failed to mmap resource in shm region (handle_type={:#x})",
export.handle_type
);
return Err(ErrUnspec);
}
} else {
Expand Down
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