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.
Jason
