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SubjectRe: [PATCH v9 05/12] vfio: Introduce common function to add capabilities
On Tue, 25 Oct 2016 02:57:58 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 10/21/2016 12:54 AM, Alex Williamson wrote:
> > On Tue, 18 Oct 2016 02:52:05 +0530
> > Kirti Wankhede <kwankhede@nvidia.com> wrote:
> >
> >> Vendor driver using mediated device framework should use
> >> vfio_info_add_capability() to add capabilities.
> >> Introduced this function to reduce code duplication in vendor drivers.
> >>
> >> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> >> Signed-off-by: Neo Jia <cjia@nvidia.com>
> >> Change-Id: I6fca329fa2291f37a2c859d0bc97574d9e2ce1a6
> >> ---
> >> drivers/vfio/vfio.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >> include/linux/vfio.h | 4 +++
> >> 2 files changed, 82 insertions(+)
> >>
> >> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> >> index a5a210005b65..e96cb3f7a23c 100644
> >> --- a/drivers/vfio/vfio.c
> >> +++ b/drivers/vfio/vfio.c
> >> @@ -1799,6 +1799,84 @@ void vfio_info_cap_shift(struct vfio_info_cap *caps, size_t offset)
> >> }
> >> EXPORT_SYMBOL_GPL(vfio_info_cap_shift);
> >>
> >> +static int sparse_mmap_cap(struct vfio_info_cap *caps, void *cap_type)
> >> +{
> >> + struct vfio_info_cap_header *header;
> >> + struct vfio_region_info_cap_sparse_mmap *sparse_cap, *sparse = cap_type;
> >> + size_t size;
> >> +
> >> + size = sizeof(*sparse) + sparse->nr_areas * sizeof(*sparse->areas);
> >> + header = vfio_info_cap_add(caps, size,
> >> + VFIO_REGION_INFO_CAP_SPARSE_MMAP, 1);
> >> + if (IS_ERR(header))
> >> + return PTR_ERR(header);
> >> +
> >> + sparse_cap = container_of(header,
> >> + struct vfio_region_info_cap_sparse_mmap, header);
> >> + sparse_cap->nr_areas = sparse->nr_areas;
> >> + memcpy(sparse_cap->areas, sparse->areas,
> >> + sparse->nr_areas * sizeof(*sparse->areas));
> >> + return 0;
> >> +}
> >> +
> >> +static int region_type_cap(struct vfio_info_cap *caps, void *cap_type)
> >> +{
> >> + struct vfio_info_cap_header *header;
> >> + struct vfio_region_info_cap_type *type_cap, *cap = cap_type;
> >> +
> >> + header = vfio_info_cap_add(caps, sizeof(*cap),
> >> + VFIO_REGION_INFO_CAP_TYPE, 1);
> >> + if (IS_ERR(header))
> >> + return PTR_ERR(header);
> >> +
> >> + type_cap = container_of(header, struct vfio_region_info_cap_type,
> >> + header);
> >> + type_cap->type = cap->type;
> >> + type_cap->subtype = cap->subtype;
> >> + return 0;
> >> +}
> >> +
> >> +int vfio_info_add_capability(struct vfio_region_info *info,
> >> + struct vfio_info_cap *caps,
> >> + int cap_type_id,
> >> + void *cap_type)
> >> +{
> >> + int ret;
> >> +
> >> + if (!cap_type)
> >> + return 0;
> >> +
> >> + switch (cap_type_id) {
> >> + case VFIO_REGION_INFO_CAP_SPARSE_MMAP:
> >> + ret = sparse_mmap_cap(caps, cap_type);
> >> + if (ret)
> >> + return ret;
> >> + break;
> >> +
> >> + case VFIO_REGION_INFO_CAP_TYPE:
> >> + ret = region_type_cap(caps, cap_type);
> >> + if (ret)
> >> + return ret;
> >> + break;
> >> + default:
> >> + return -EINVAL;
> >> + }
> >> +
> >> + info->flags |= VFIO_REGION_INFO_FLAG_CAPS;
> >> +
> >> + if (caps->size) {
> >> + if (info->argsz < sizeof(*info) + caps->size) {
> >> + info->argsz = sizeof(*info) + caps->size;
> >> + info->cap_offset = 0;
> >> + } else {
> >> + vfio_info_cap_shift(caps, sizeof(*info));
> >> + info->cap_offset = sizeof(*info);
> >
> > This doesn't work. We build the capability chain in a buffer and
> > vfio_info_cap_add() expects the chain to be zero-based as each
> > capability is added. vfio_info_cap_shift() is meant to be called once
> > on that buffer immediately before copying it back to the user buffer to
> > adjust the chain offsets to account for the offset within the buffer.
> > vfio_info_cap_shift() cannot be called repeatedly on the buffer as we
> > do support multiple capabilities in a chain.
> >
>
> From the code I see, we add one type of capability at a time, either
> VFIO_REGION_INFO_CAP_SPARSE_MMAP or VFIO_REGION_INFO_CAP_TYPE. Both are
> not the part of same case in the switch, right?
> I do tested VFIO_REGION_INFO_CAP_SPARSE_MMAP by mapping some part of
> BAR0 and that works.

That simply means that we don't _currently_ have a user that implements
multiple chain entries. The interface is however designed to support
multiple entries and this breaks that goal. Thanks,

Alex

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