[PATCH 06/23] drm/xe/svm: Introduce a helper to build sg table from hmm range

[Jason Gunthorpe]

On Thu, May 02, 2024 at 11:11:04AM +0200, Thomas Hellström wrote:

> It's true the cpu vma lookup is a remnant from amdkfd. The idea here is
> to replace that with fixed prefaulting ranges of tunable size. So far,
> as you mention, the prefaulting range has been determined by the CPU
> vma size. Given previous feedback, this is going to change.

Perhaps limiting prefault to a VMA barrier is a reasonable thing to
do, but the implementation should be pushed into hmm_range_fault and
not open coded in the driver.

> Still the prefaulting range needs to be restricted to avoid -EFAULT
> failures in hmm_range_fault(). That can ofc be done by calling it
> without HMM_PFN_REQ_FAULT for the range and interpret the returned
> pnfs. 

Yes, this is exactly what that feature is for, you mark your prefetch
differently from the fault critical page(s).

> There is a performance concern of this approach as compared to
> peeking at the CPU vmas directly, since hmm_range_fault() would need to
> be called twice. Any guidelines ideas here?

If there is something wrong with hmm_range_fault() then please fix
it. I'm not sure why you'd call it twice, the HMM_PFN_REQ_FAULT is per
PFN?

Jason