Re: i915.ko WC writes are slow after ea8596bb2d8d379

[H. Peter Anvin]

On 10/08/2014 12:49 PM, Chris Wilson wrote:
> 
> Indeed, this appears to be the explanation. (And here I thought PAT
> superseded mtrrs - i915.ko stopped trying to use assign an mtrr for its
> GTT quite a while ago.)
> 
> Replacing the stop_machine there with on_each_cpu does the trick:
> 

It should, but there seem to be quite a few drivers which still muck
with MTRRs.  However, i915 is not one of them, it calls
io_mapping_create_wc() followed by arch_phys_wc_add(), so I'm wondering
what the heck is going on here.

> Naively I would say that we lost the wc on our ioremap.
> /sys/kernel/debug/x86/pat_memtype_list remained the same across repeated
> runs.

Could you tell me what the above looks like?

	-hpa




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