On 08/15/2014 12:21 PM, Ville Syrjälä wrote:
On Thu, Aug 14, 2014 at 05:55:11AM +0200, Juergen Gross wrote:
On 08/13/2014 05:07 PM, Jesse Barnes wrote:
On Fri, 8 Aug 2014 15:14:15 +0200
Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
Adding relevant mailing lists.
On Fri, Aug 8, 2014 at 1:23 PM, Juergen Gross <jgross@xxxxxxxx> wrote:
I'm just about to create a patch for full PAT support in the Linux
kernel, including Xen. For this purpose I introduce a translation
between cache modes and pte bits.
Scanning the kernel sources for usage of the cache mode bits in the
pte I discovered drivers/gpu/drm/i915/i915_gem_gtt.h is using
_PAGE_PCD, _PAGE_PWT and _PAGE_PAT. I think those defines are used
to create ptes not for usage by the main processor, but for the
graphics processor. Is this true? In this case I'd suggest to define
i915-specific macros instead of using the x86 ones.
Yeah, those are gpu specific PAT tables, but the hw engineers
specifically designed this to match, and we've tried to follow the cpu
side to match it. Especially in the future that will be somewhat
important, since we want to fully share the entire address space
between cpu and gpu on the next platform. Jesse is working on that.
Right, we have an x86 compatible MMU in the GPU itself, so re-using the
defines makes sense. I suppose with your work you'll move them and
make them a bit more opaque? If so, we'll still want a way to get at
them directly, or access your mapping functions for generating PTE bits
for the GPU MMU.
Using the mapping functions I'm introducing should work, if the MMU has
an x86 compatible MSR_IA32_CR_PAT which is configured the same way as
on the x86 processor (be aware that Xen is using another MSR_IA32_CR_PAT
setting as the Linux kernel).
We have a PAT that is structured the same way as the x86 PAT. But the
contents of the PAT entries are obviously specific to the GPU so it's
not identical. But the pcd/pwt/pat bits index the PAT in exactly the
same way as on x86.
See bdw_setup_private_ppat() and chv_setup_private_ppat() for how we
set up the PAT.
So you are using the PAT bit in the ptes, but the semantic for the GPU
will be different as for the x86 processor, because the GPU PAT is set
up differently from the x86 one.
In case you are sharing ptes between GPU and x86 processor in future,
this might lead to problems when the x86 processor will use ptes with
the PAT bit set.
Juergen
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