linux-kernel - Re: runtime regression with "x86/mm/pat: Emulate PAT when it is disabled"

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Date:	Thu, 10 Mar 2016 09:42:51 -0500
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	Toshi Kani <toshi.kani@....com>
CC:	Borislav Petkov <bp@...e.de>,
	Richard Purdie <richard.purdie@...uxfoundation.org>,
	Toshi Kani <toshi.kani@...com>,
	Bruce Ashfield <bruce.ashfield@...driver.com>,
	"Hart, Darren" <darren.hart@...el.com>,
	"saul.wold" <saul.wold@...el.com>, <linux-kernel@...r.kernel.org>
Subject: Re: runtime regression with "x86/mm/pat: Emulate PAT when it is
 disabled"

[Re: runtime regression with "x86/mm/pat: Emulate PAT when it is disabled"] On 07/03/2016 (Mon 18:35) Toshi Kani wrote:

> On Mon, 2016-03-07 at 17:56 -0700, Toshi Kani wrote:

[...]

> And also an output of /proc/cpuinfo, please?

Here is the output of /proc/cpuinfo in the guest session, while
running on pentium dual core as host (no vmx):


processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 6
model name	: QEMU Virtual CPU version 2.5+
stepping	: 3
cpu MHz		: 2593.449
cache size	: 4096 KB
physical id	: 0
siblings	: 1
core id		: 0
cpu cores	: 1
apicid		: 0
initial apicid	: 0
fdiv_bug	: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 4
wp		: yes
flags		: fpu de pse tsc msr pae mce cx8 apic sep pge cmov mmx fxsr sse sse2 pni hypervisor
bugs		:
bogomips	: 5186.89
clflush size	: 32
cache_alignment	: 32
address sizes	: 36 bits physical, 32 bits virtual
power management:

Paul.
--

> 
> I think I know what's going on.  I noticed that you have the following
> message in your dmesg files.
> 
>  [    0.000000] MTRR: Disabled
> 
> MTRR is set to disabled when your CPU is Intel but does not support MTRR.
>  Perhaps, QEMU does not emulate MTRR?
> 
> pat_init() is not called when MTRR is disabled.  I think this dependency is
> wrong, and it needs to be fixed.
> 
> This issue has been there for a long time, and you have been running
> essentially as PAT disabled in the past.  The commit in question simply
> detected this issue.
> 
> Thanks,
> -Toshi