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

[Thomas Hellström]

On Thu, 2024-05-02 at 09:46 -0300, Jason Gunthorpe wrote:
> 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?

Ah, yes you're right. I somehow thought it was per range. Makes sense
now.

Thanks,
Thomas



> 
> Jason