Hi Akihiko, > Subject: Re: [PATCH v7 10/10] virtio-gpu-dmabuf: Create dmabuf for blobs > associated with VFIO devices > > >> > >> On 2026/02/23 17:00, Vivek Kasireddy wrote: > >>> In addition to memfd, a blob resource can also have its backing > >>> storage in a VFIO device region. Since, there is no effective way > >>> to determine where the backing storage is located, we first try to > >>> create a dmabuf assuming it is in memfd. If that fails, we try to > >>> create a dmabuf assuming it is in VFIO device region. > >>> > >>> So, we first call virtio_gpu_create_udmabuf() to check if the blob's > >>> backing storage is located in a memfd or not. If it is not, we invoke > >>> the vfio_device_create_dmabuf() API which identifies the right VFIO > >>> device and eventually creates a dmabuf fd. > >>> > >>> Note that in virtio_gpu_remap_dmabuf(), we first try to test if > >>> the VFIO dmabuf exporter supports mmap or not. If it doesn't, we > >>> fall back to the vfio_device_mmap_dmabuf() API to get a mapping > >>> created for the dmabuf. > >>> > >>> 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/meson.build | 1 + > >>> hw/display/vfio-dmabuf-stubs.c | 23 +++++++++++++++++++++++ > >>> hw/display/virtio-gpu-dmabuf.c | 32 ++++++++++++++++++++++++-- > --- > >> --- > >>> 3 files changed, 48 insertions(+), 8 deletions(-) > >>> create mode 100644 hw/display/vfio-dmabuf-stubs.c > >>> > >>> diff --git a/hw/display/meson.build b/hw/display/meson.build > >>> index f5f92b1068..7d2bd839cc 100644 > >>> --- a/hw/display/meson.build > >>> +++ b/hw/display/meson.build > >>> @@ -70,6 +70,7 @@ if > >> config_all_devices.has_key('CONFIG_VIRTIO_GPU') > >>> if_true: [files('virtio-gpu-base.c', > >>> 'virtio-gpu.c'), pixman]) > >>> if host_os == 'linux' > >>> virtio_gpu_ss.add(files('virtio-gpu-dmabuf.c')) > >>> + virtio_gpu_ss.add(when: 'CONFIG_VFIO', if_false: files('vfio- > >> dmabuf-stubs.c')) > >>> else > >>> virtio_gpu_ss.add(files('virtio-gpu-dmabuf-stubs.c')) > >>> endif > >>> diff --git a/hw/display/vfio-dmabuf-stubs.c b/hw/display/vfio- > dmabuf- > >> stubs.c > >>> new file mode 100644 > >>> index 0000000000..12f016f0c5 > >>> --- /dev/null > >>> +++ b/hw/display/vfio-dmabuf-stubs.c > >>> @@ -0,0 +1,23 @@ > >>> +/* > >>> + * Copyright (c) 2026 Intel and/or its affiliates. > >>> + * > >>> + * SPDX-License-Identifier: GPL-2.0-or-later > >>> + */ > >>> + > >>> +#include "qemu/osdep.h" > >>> +#include "qemu/error-report.h" > >>> +#include "qemu/iov.h" > >>> + > >>> +bool vfio_device_create_dmabuf(struct iovec *iov, unsigned int > >> iov_cnt, > >>> + int *fd, Error **errp) > >>> +{ > >>> + error_setg(errp, "VFIO dmabuf support is not enabled"); > >>> + return false; > >>> +} > >>> + > >>> +bool vfio_device_mmap_dmabuf(struct iovec *iov, unsigned int > >> iov_cnt, > >>> + void **addr, size_t total_size, Error > >>> **errp) > >>> +{ > >>> + error_setg(errp, "VFIO mmap dmabuf support is not enabled"); > >>> + return false; > >>> +} > >> > >> This is VFIO's implementation and shouldn't be here. Follow what > >> hw/vfio/iommufd-stubs.c, the previously-mentioned example, does. > > Ok, I'll move this file to hw/vfio directory. > > > >> > >>> diff --git a/hw/display/virtio-gpu-dmabuf.c b/hw/display/virtio-gpu- > >> dmabuf.c > >>> index fdcf8e53a2..3f39371de1 100644 > >>> --- a/hw/display/virtio-gpu-dmabuf.c > >>> +++ b/hw/display/virtio-gpu-dmabuf.c > >>> @@ -18,6 +18,7 @@ > >>> #include "ui/console.h" > >>> #include "hw/virtio/virtio-gpu.h" > >>> #include "hw/virtio/virtio-gpu-pixman.h" > >>> +#include "hw/vfio/vfio-device.h" > >>> #include "trace.h" > >>> #include "system/ramblock.h" > >>> #include "system/hostmem.h" > >>> @@ -82,13 +83,21 @@ virtio_gpu_create_udmabuf(struct > >> virtio_gpu_simple_resource *res, int *fd, > >>> static bool virtio_gpu_remap_dmabuf(struct > >> virtio_gpu_simple_resource *res, > >>> Error **errp) > >>> { > >>> + Error *mmap_err = NULL; > >>> void *map; > >>> > >>> map = mmap(NULL, res->blob_size, PROT_READ, MAP_SHARED, > >> res->dmabuf_fd, 0); > >>> if (map == MAP_FAILED) { > >>> - error_setg_errno(errp, errno, "dmabuf mmap failed"); > >>> + error_setg_errno(&mmap_err, errno, "dmabuf mmap failed: "); > >>> res->remapped = NULL; > >>> - return false; > >>> + > >>> + if (!vfio_device_mmap_dmabuf(res->iov, res->iov_cnt, &map, > >>> + res->blob_size, errp)) { > >>> + error_report_err(mmap_err); > >>> + return false; > >>> + } > >>> + error_free(mmap_err); > >>> + mmap_err = NULL; > >>> } > >>> res->remapped = map; > >>> return true; > >>> @@ -145,7 +154,7 @@ bool virtio_gpu_have_udmabuf(void) > >>> > >>> void virtio_gpu_init_dmabuf(struct virtio_gpu_simple_resource > *res) > >>> { > >>> - Error *local_err = NULL; > >>> + Error *local_err = NULL, *vfio_err = NULL; > >>> VirtIOGPUErrorType err; > >>> void *pdata = NULL; > >>> int fd; > >>> @@ -158,12 +167,19 @@ void virtio_gpu_init_dmabuf(struct > >> virtio_gpu_simple_resource *res) > >>> err = virtio_gpu_create_udmabuf(res, &fd, &local_err); > >>> if (err != VIRTIO_GPU_NO_ERROR) { > >>> error_prepend(&local_err, "Cannot create dmabuf: "); > >>> - if (err == VIRTIO_GPU_GUEST_ERROR) { > >>> - qemu_log_mask(LOG_GUEST_ERROR, > >>> - "Cannot create dmabuf: incompatible > >>> mem\n"); > >>> + > >>> + if (!vfio_device_create_dmabuf(res->iov, res->iov_cnt, &fd, > >>> + &vfio_err)) { > >>> + if (err == VIRTIO_GPU_GUEST_ERROR) { > >>> + qemu_log_mask(LOG_GUEST_ERROR, > >>> + "Cannot create dmabuf: incompatible > >>> mem\n"); > >> > >> This logic is faulty. err is not set by vfio_device_create_dmabuf(), so > >> it reports a guest error even when the memory is compatible with > >> vfio_device_create_dmabuf() and it raised an error for a different > >> reason. > > Maybe I misunderstood your previous comment about this but I > thought > > we are going to assume that if both virtio_gpu_create_udmabuf() and > > vfio_device_create_dmabuf() fail (assuming Guest error in case of > > virtio_gpu_create_udmabuf ()) then it is reasonable to assume that > Guest > > is at fault for passing in incompatible memory addresses. > > It's not what I suggested. What I suggested is as follows: > > virtio_gpu_create_udmabuf(res); > > if (it turned out the memory is incompatible with udmabuf) { > > virtio_gpu_create_vfio_dmabuf(res); > > if (it turned out the memory is not backed by VFIO) { > > qemu_log_mask(LOG_GUEST_ERROR, "incompatible\n"); > > return; > > } > > } > > https://lore.kernel.org/qemu-devel/c7c03384-4815-4032-89bd- > [email protected]/ > > It is not correct to assume vfio_device_create_dmabuf() is failed due to > a guest error when virtio_gpu_create_udmabuf(). For example, when > mmap() > fails in vfio_device_mmap_dmabuf(), it is a host error, and reporting a > guest error in the case is a bug. > > > > > However, in order to fix this properly, I think we would have to have > > vfio_device_create_dmabuf() return VIRTIO_GPU_GUEST/HOST_ERROR > > (or something equivalent) which I was trying to avoid. Do you see any > > other proper way to fix this issue without messing with the return type > > of vfio_device_create_dmabuf()? > > > > The reason why I am hesitant to make vfio_device_create_dmabuf() > return > > Guest/Host error is because a future user of this API may not need this > info. > > A future user may not need it, but we need it to make this code bug-free. Ok, I plan to add the following enum and make it available to VFIO and virtio-gpu-dmabuf.c: typedef enum DmabufCreateErrorType { DMABUF_CREATE_NO_ERROR = 1, /* Guest is responsible for this error */ DMABUF_CREATE_GUEST_ERROR = -1, /* Host is at fault for this error */ DMABUF_CREATE_HOST_ERROR = -2, } DmabufCreateErrorType;
Is the naming OK? And, what would be a good location (header) to have this in? Thanks, Vivek > > Regards, > Akihiko Odaki > > > > > Thanks, > > Vivek > > > >> > >> Regards, > >> Akihiko Odaki > >> > >>> + } > >>> + error_report_err(local_err); > >>> + error_report_err(vfio_err); > >>> + return; > >>> } > >>> - error_report_err(local_err); > >>> - return; > >>> + error_free(local_err); > >>> + local_err = NULL; > >>> } > >>> > >>> res->dmabuf_fd = fd; > >
