Re: [PATCH] drm/amdgpu: Fix default page access routing

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On 2015-11-06 02:28, Christian König wrote:
On 05.11.2015 20:06, Jay Cornwall wrote:
The VM default page (used when a VM translation fails) is allocated in
system memory. The VM is misconfigured to interpret the physical address
as referencing a VRAM physical page.

Route default page accesses to system memory.

Signed-off-by: Jay Cornwall <jay@xxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v4.2+

Nice catch, patch is Reviewed-by: Christian König <christian.koenig@xxxxxxx>

Do we also need this for Radeon?

It looks like this field was introduced in CIK. This change could be replicated in radeon on those parts, but I haven't got one to test with.

I'm not sure if this misconfiguration is fatal. The default page should be used for loads only. I haven't been able to get anything besides 0 from a faulting load on my Fiji with a system or VRAM default page. A physical address > VRAM doesn't seem to cause problems, either.

My primary motivation for this change is to allow HSA to handle faulting atomic operations. The KFD exposes a MTYPE=UC aperture to the user which allows atomics to be forwarded to system memory, via MC/BIF/PCI. When one of these atomic operations faults the MC is unable to handle the VRAM atomic (atomics are implemented by TC) and the TC hangs waiting for a response.

If the default page is directed towards BIF then the atomic is correctly discarded.

--
Jay Cornwall
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