On Mon, Mar 16, 2020 at 07:49:35PM +0100, Christoph Hellwig wrote:
> On Mon, Mar 16, 2020 at 11:42:19AM -0700, Ralph Campbell wrote:
> >
> > On 3/16/20 10:52 AM, Christoph Hellwig wrote:
> >> No driver has actually used properly wire up and support this feature.
> >> There is various code related to it in nouveau, but as far as I can tell
> >> it never actually got turned on, and the only changes since the initial
> >> commit are global cleanups.
> >
> > This is not actually true. OpenCL 2.x does support SVM with nouveau and
> > device private memory via clEnqueueSVMMigrateMem().
> > Also, Ben Skeggs has accepted a set of patches to map GPU memory after being
> > migrated and this change would conflict with that.
>
> Can you explain me how we actually invoke this code?
>
> For that we'd need HMM_PFN_DEVICE_PRIVATE NVIF_VMM_PFNMAP_V0_VRAM
> set in ->pfns before calling hmm_range_fault, which isn't happening.
Oh, I got tripped on this too
The logic is backwards from what you'd think.. If you *set*
HMM_PFN_DEVICE_PRIVATE then this triggers:
hmm_pte_need_fault():
if ((cpu_flags & range->flags[HMM_PFN_DEVICE_PRIVATE])) {
/* Do we fault on device memory ? */
if (pfns & range->flags[HMM_PFN_DEVICE_PRIVATE]) {
*write_fault = pfns & range->flags[HMM_PFN_WRITE];
*fault = true;
}
return;
}
Ie if the cpu page is a DEVICE_PRIVATE and the caller sets
HMM_PFN_DEVICE_PRIVATE in the input flags (pfns) then it always faults
it and never sets HMM_PFN_DEVICE_PRIVATE in the output flags.
So setting 0 enabled device_private support, and nouveau is Ok.
AMDGPU is broken because it can't handle device private and can't set
the flag to supress it.
I was going to try and sort this out as part of getting rid of range->flags
Jason
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