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

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

Hi Cedric,

> 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.
> 
> 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.

> 
> 
> 
> >       }
> >
> >       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));

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;