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

[Toshi Kani <toshi.kani@hpe.com>]

On Thu, 2016-03-10 at 09:42 -0500, Paul Gortmaker wrote:
> [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:

This confirms the issue - QEMU's virtual Intel CPU does not support MTRR. 

When MTRR is disabled, the kernel does not call pat_init().  pat_enabled()
is still set to true when CONFIG_X86_PAT is set.  CONFIG_X86_PAT depends on
CONFIG_MTRR, and assumes that MTRR is enabled. 

Thanks,
-Toshi

> 
> 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