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.
Thanks,
Alex
