linux-kernel - Re: i915.ko WC writes are slow after ea8596bb2d8d379

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Message-ID: <5435AE71.60304@linux.intel.com>
Date:	Wed, 08 Oct 2014 14:36:49 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>,
	Chuck Ebbert <cebbert.lkml@...il.com>
CC:	linux-kernel@...r.kernel.org,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jason Baron <jbaron@...mai.com>, yrl.pp-manager.tt@...achi.com,
	Borislav Petkov <bpetkov@...e.de>,
	Ingo Molnar <mingo@...nel.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: i915.ko WC writes are slow after ea8596bb2d8d379

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 superseded mtrrs - i915.ko stopped trying to use assign an mtrr for its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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