linux-kernel - Re: [PATCH v9 05/12] vfio: Introduce common function to add capabilities

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Date:   Mon, 24 Oct 2016 15:39:19 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Kirti Wankhede <kwankhede@...dia.com>
Cc:     <pbonzini@...hat.com>, <kraxel@...hat.com>, <cjia@...dia.com>,
        <qemu-devel@...gnu.org>, <kvm@...r.kernel.org>,
        <kevin.tian@...el.com>, <jike.song@...el.com>,
        <bjsdjshi@...ux.vnet.ibm.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 05/12] vfio: Introduce common function to add
 capabilities

On Tue, 25 Oct 2016 02:57:58 +0530
Kirti Wankhede <kwankhede@...dia.com> wrote:

> On 10/21/2016 12:54 AM, Alex Williamson wrote:
> > On Tue, 18 Oct 2016 02:52:05 +0530
> > Kirti Wankhede <kwankhede@...dia.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@...dia.com>
> >> Signed-off-by: Neo Jia <cjia@...dia.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