RE: [PATCH v12 09/10] virtio-gpu-dmabuf: Improve error handling with 'Error **' and err enum

["Kasireddy, Vivek" <vivek.kasireddy@intel.com>]

Hi Akihiko,

> Subject: Re: [PATCH v12 09/10] virtio-gpu-dmabuf: Improve error
> handling with 'Error **' and err enum
> >>
> >> On 3/19/26 06:15, Vivek Kasireddy wrote:
> >>> Make the error handling more robust in virtio_gpu_init_udmabuf()
> >>> by introducing 'Error **' parameter to capture errors and using
> >>> an enum from VFIO to categorize different errors. This allows for
> >>> better error reporting and handling of errors from
> >>> virtio_gpu_create_udmabuf() and virtio_gpu_remap_dmabuf().
> >>>
> >>> Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
> >>> Cc: Alex Bennée <alex.bennee@linaro.org>
> >>> Cc: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
> >>> Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> >>> Cc: Alex Williamson <alex@shazbot.org>
> >>> Cc: Cédric Le Goater <clg@redhat.com>
> >>> Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
> >>> Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
> >>> ---
> >>>    hw/display/virtio-gpu-dmabuf.c | 67 +++++++++++++++++++++++--
> -----
> >> ----
> >>>    1 file changed, 45 insertions(+), 22 deletions(-)
> >>>
> >>> diff --git a/hw/display/virtio-gpu-dmabuf.c b/hw/display/virtio-gpu-
> >> dmabuf.c
> >>> index e35f7714a9..89aa487654 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"
> >>> @@ -27,16 +28,18 @@
> >>>    #include "standard-headers/linux/udmabuf.h"
> >>>    #include "standard-headers/drm/drm_fourcc.h"
> >>>
> >>> -static void virtio_gpu_create_udmabuf(struct
> >> virtio_gpu_simple_resource *res)
> >>> +static int virtio_gpu_create_udmabuf(struct
> >> virtio_gpu_simple_resource *res,
> >>> +                                     Error **errp)
> >>>    {
> >>>        g_autofree struct udmabuf_create_list *list = NULL;
> >>>        RAMBlock *rb;
> >>>        ram_addr_t offset;
> >>> -    int udmabuf, i;
> >>> +    int udmabuf, i, fd;
> >>>
> >>>        udmabuf = udmabuf_fd();
> >>>        if (udmabuf < 0) {
> >>> -        return;
> >>> +        error_setg(errp, "udmabuf device not available or enabled");
> >>> +        return VFIO_DMABUF_CREATE_ERR_UNSPEC;
> >>
> >> The function virtio_gpu_create_udmabuf() is returning
> VFIO_DMABUF_*
> >> enum
> >> values, which is problematic because the function creates a
> udmabuf,
> >> not
> >> a VFIO dmabuf.
> >>
> >> This creates a layering violation. The virtio-gpu-dmabuf code (which
> >> handles both udmabuf and VFIO dmabuf creation) is using error
> codes
> >> defined in the VFIO-specific header.
> 
> The rationale of using error codes is as follows:
> 
>  > In that case, using it for udmabuf is cheating, but I think it's fine
>  > since it's locally-contained in virtio-gpu-dmabuf.c, its intent is
>  > still clear, and it has no adverse effect for users (at least there
>  > will be no bug).
> 
> https://lore.kernel.org/qemu-devel/becde56b-90bd-40ca-9329-
> 0c92b9519a67@rsg.ci.i.u-tokyo.ac.jp/
> 
> >>
> >> Please find another solution.
> > Other solutions I can think of are either move these error enums into
> virtio-gpu
> > (and disregard the error return type from vfio) or move them to some
> other header
> > where they are visible to both virtio-gpu and vfio. I'd like hear
> Akihiko's thoughts/
> > comments on how to proceed given that he had reviewed virtio-gpu
> patches in
> > this series.
> 
> Disregarding the error conditions of VFIO is not OK. That can cause a
> guest error to be reported as a host error.
> 
> I think the simplest alternative is just to have a duplicate enum for
> virtio-gpu.
That would address the layering violation but Cedric's other concern is
that he believes having errp is sufficient and using these error enums in
VFIO would be overkill.

Thanks,
Vivek
> 
> >
> >>
> >>
> >>
> >>>        }
> >>>
> >>>        list = g_malloc0(sizeof(struct udmabuf_create_list) +
> >>> @@ -45,7 +48,8 @@ static void virtio_gpu_create_udmabuf(struct
> >> virtio_gpu_simple_resource *res)
> >>>        for (i = 0; i < res->iov_cnt; i++) {
> >>>            rb = qemu_ram_block_from_host(res->iov[i].iov_base, false,
> >> &offset);
> >>>            if (!rb || rb->fd < 0) {
> >>> -            return;
> >>> +            error_setg(errp, "IOV memory address incompatible with
> >> udmabuf ");
> >>> +            return VFIO_DMABUF_CREATE_ERR_INVALID_IOV;
> >>>            }
> >>>
> >>>            list->list[i].memfd  = rb->fd;
> >>> @@ -56,22 +60,28 @@ static void
> virtio_gpu_create_udmabuf(struct
> >> virtio_gpu_simple_resource *res)
> >>>        list->count = res->iov_cnt;
> >>>        list->flags = UDMABUF_FLAGS_CLOEXEC;
> >>>
> >>> -    res->dmabuf_fd = ioctl(udmabuf, UDMABUF_CREATE_LIST, list);
> >>> -    if (res->dmabuf_fd < 0) {
> >>> -        warn_report("%s: UDMABUF_CREATE_LIST: %s", __func__,
> >>> -                    strerror(errno));
> >>> +    fd = ioctl(udmabuf, UDMABUF_CREATE_LIST, list);
> >>> +    if (fd < 0) {
> >>> +        error_setg_errno(errp, errno, "UDMABUF_CREATE_LIST: ioctl
> >> failed");
> >>> +        if (errno == EINVAL || errno == EBADFD) {
> >>> +            return VFIO_DMABUF_CREATE_ERR_INVALID_IOV;
> >>> +        }
> >>> +        return VFIO_DMABUF_CREATE_ERR_UNSPEC;
> >>>        }
> >>> +    return fd;
> >>>    }
> >>>
> >>> -static void virtio_gpu_remap_dmabuf(struct
> >> virtio_gpu_simple_resource *res)
> >>> +static void *virtio_gpu_remap_dmabuf(struct
> >> virtio_gpu_simple_resource *res,
> >>> +                                     Error **errp)
> >>>    {
> >>> -    res->remapped = mmap(NULL, res->blob_size, PROT_READ,
> >>> -                         MAP_SHARED, res->dmabuf_fd, 0);
> >>> -    if (res->remapped == MAP_FAILED) {
> >>> -        warn_report("%s: dmabuf mmap failed: %s", __func__,
> >>> -                    strerror(errno));
> >>> -        res->remapped = 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");
> >>> +        return NULL;
> >>>        }
> >>> +    return map;
> >>>    }
> >>>
> >>>    static void virtio_gpu_destroy_dmabuf(struct
> >> virtio_gpu_simple_resource *res)
> >>> @@ -125,22 +135,35 @@ bool virtio_gpu_have_udmabuf(void)
> >>>
> >>>    void virtio_gpu_init_dmabuf(struct virtio_gpu_simple_resource
> *res)
> >>>    {
> >>> +    Error *local_err = NULL;
> >>>        void *pdata = NULL;
> >>>
> >>> -    res->dmabuf_fd = -1;
> >>>        if (res->iov_cnt == 1 &&
> >>>            res->iov[0].iov_len < 4096) {
> >>> +        res->dmabuf_fd = -1;
> >>>            pdata = res->iov[0].iov_base;
> >>>        } else {
> >>> -        virtio_gpu_create_udmabuf(res);
> >>> +        res->dmabuf_fd = virtio_gpu_create_udmabuf(res,
> &local_err);
> >>> +        if (res->dmabuf_fd ==
> >> VFIO_DMABUF_CREATE_ERR_INVALID_IOV) {
> >>> +            error_free_or_abort(&local_err);
> >>
> >> Why not report the error in the QEMU log below ?
> > I think the idea is that in the case of INVALID_IOV error, it is sufficient
> > to just report that the Guest passed in incompatible memory
> addresses.
> > But I guess we could also just do:
> > qemu_log_mask(LOG_GUEST_ERROR, "%s\n",
> error_get_pretty(local_err));
> 
> The error message may not be useful. e.g., "UDMABUF_CREATE_LIST:
> ioctl
> failed" does not tell what error the guest made and how to fix it.
> 
> Regards,
> Akihiko Odaki
> 
> >
> > Thanks,
> > Vivek
> >>
> >>> +
> >>> +            qemu_log_mask(LOG_GUEST_ERROR,
> >>> +                          "Cannot create dmabuf: incompatible memory\n");
> >>> +            return;
> >>> +        } else if (res->dmabuf_fd >= 0) {
> >>> +            pdata = virtio_gpu_remap_dmabuf(res, &local_err);
> >>> +            if (!pdata) {
> >>> +                virtio_gpu_destroy_dmabuf(res);
> >>> +            }
> >>> +        } else {
> >>> +            res->dmabuf_fd = -1;
> >>> +        }
> >>> +
> >>>            if (res->dmabuf_fd < 0) {
> >>> +            error_report_err(local_err);
> >>>                return;
> >>>            }
> >>> -        virtio_gpu_remap_dmabuf(res);
> >>> -        if (!res->remapped) {
> >>> -            return;
> >>> -        }
> >>> -        pdata = res->remapped;
> >>> +        res->remapped = pdata;
> >>>        }
> >>>
> >>>        res->blob = pdata;
> >