Re: [PATCH rfc] vfio-pci: Allow write combining

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 1 Aug 2024 11:19:14 -0300
Jason Gunthorpe <jgg@ziepe.ca> wrote:

> On Wed, Jul 31, 2024 at 08:53:52AM -0700, Keith Busch wrote:
> > From: Keith Busch <kbusch@kernel.org>
> > 
> > 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