On Thu, Aug 01, 2024 at 09:41:23AM -0600, Alex Williamson wrote:
> On Thu, 1 Aug 2024 11:19:14 -0300
> Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> > On Wed, Jul 31, 2024 at 08:53:52AM -0700, Keith Busch wrote:
> > > From: Keith Busch <kbusch@xxxxxxxxxx>
> > >
> > > Write combining can be provide performance improvement for places that
> > > can safely use this capability.
> > >
> > > Previous discussions on the topic suggest a vfio user needs to
> > > explicitly request such a mapping, and it sounds like a new vfio
> > > specific ioctl to request this is one way recommended way to do that.
> > > This patch implements a new ioctl to achieve that so a user can request
> > > write combining on prefetchable memory. A new ioctl seems a bit much for
> > > just this purpose, so the implementation here provides a "flags" field
> > > with only the write combine option defined. The rest of the bits are
> > > reserved for future use.
> >
> > This is a neat hack for sure
> >
> > But how about adding this flag to vfio_region_info ?
> >
> > @@ -275,6 +289,7 @@ struct vfio_region_info {
> > #define VFIO_REGION_INFO_FLAG_WRITE (1 << 1) /* Region supports write */
> > #define VFIO_REGION_INFO_FLAG_MMAP (1 << 2) /* Region supports mmap */
> > #define VFIO_REGION_INFO_FLAG_CAPS (1 << 3) /* Info supports caps */
> > +#define VFIO_REGION_INFO_REQ_WC (1 << 4) /* Request a write combining mapping*/
> > __u32 index; /* Region index */
> > __u32 cap_offset; /* Offset within info struct of first cap */
> > __aligned_u64 size; /* Region size (bytes) */
> >
> >
> > It specify REQ_WC when calling VFIO_DEVICE_GET_REGION_INFO
> >
> > The kernel will then return an offset value that yields a WC
> > mapping. It doesn't displace the normal non-WC mapping?
> >
> > Arguably we should fixup the kernel to put the mmap cookies into a
> > maple tree so they can be dynamically allocated and more densely
> > packed.
>
> vfio_region_info.flags in not currently tested for input therefore this
> proposal could lead to unexpected behavior for a caller that doesn't
> currently zero this field. It's intended as an output-only field.
Perhaps a REGION_INFO2 then?
I still think per-request is better than a global flag
Jason
