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

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Date:	Fri, 11 Mar 2016 17:28:37 -0500
From:	Bruce Ashfield <bruce.ashfield@...driver.com>
To:	Borislav Petkov <bp@...e.de>, Paolo Bonzini <pbonzini@...hat.com>
CC:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Toshi Kani <toshi.kani@....com>,
	Richard Purdie <richard.purdie@...uxfoundation.org>,
	Toshi Kani <toshi.kani@...com>,
	"Hart, Darren" <darren.hart@...el.com>,
	"saul.wold" <saul.wold@...el.com>, <linux-kernel@...r.kernel.org>,
	kvm ML <kvm@...r.kernel.org>, x86-ml <x86@...nel.org>
Subject: Re: runtime regression with "x86/mm/pat: Emulate PAT when it is
 disabled"

On 2016-03-11 5:16 PM, Borislav Petkov wrote:
> On Fri, Mar 11, 2016 at 08:18:23PM +0100, Paolo Bonzini wrote:
>> Somebody got it wrong 10-ish years ago, and nobody has ever checked since.
>>
>> But, don't use qemu32 or qemu64.  Use kvm32 and kvm64, or better
>> something like the host you run on ("-cpu Nehalem", "-cpu SandyBridge",
>> "-cpu Haswell-noTSX" etc.).
>
> Paul, Richard, how about it?

I'm not Paul/Richard, but I can answer :)

We want a more generic cpu for these qemu references. Something that
doesn't vary, since it runs on any number of hosts. There are some
horrible issues we've had to solve with -cpu host in the past.

Switching to the kvm cpu type should work, plus we can add cpu
extensions on the fly as necessary.

That's definitely a valid outcome from this discussion .. a cpu
type that doesn't shoot through the middle of the expected
capabilities .. and a bonus if the kernel or qemu can be tweaked
to survive a similar mix up in the flags in the future.

Cheers,

Bruce

>
>> I really, really should fix those defaults...
>
> Here's a start, while I have everything fresh in my head.
>
> ---
> From: Borislav Petkov <bp@...e.de>
> Date: Fri, 11 Mar 2016 23:11:05 +0100
> Subject: [PATCH] target-i386/cpu: Correct MTRR and PAT feature bits
>
> Pentium Pro had MTRRs but not PAT, PAT support appeared in Pentium III.
> Fix all defines.
>
> Signed-off-by: Borislav Petkov <bp@...e.de>
> ---
>   target-i386/cpu.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> index 0f38d1eae317..fa7ea4a8c229 100644
> --- a/target-i386/cpu.c
> +++ b/target-i386/cpu.c
> @@ -308,12 +308,12 @@ static const char *cpuid_6_feature_name[] = {
>   #define PENTIUM_FEATURES (I486_FEATURES | CPUID_DE | CPUID_TSC | \
>             CPUID_MSR | CPUID_MCE | CPUID_CX8 | CPUID_MMX | CPUID_APIC)
>   #define PENTIUM2_FEATURES (PENTIUM_FEATURES | CPUID_PAE | CPUID_SEP | \
> -          CPUID_MTRR | CPUID_PGE | CPUID_MCA | CPUID_CMOV | CPUID_PAT | \
> +          CPUID_MTRR | CPUID_PGE | CPUID_MCA | CPUID_CMOV | \
>             CPUID_PSE36 | CPUID_FXSR)
> -#define PENTIUM3_FEATURES (PENTIUM2_FEATURES | CPUID_SSE)
> +#define PENTIUM3_FEATURES (PENTIUM2_FEATURES | CPUID_SSE | CPUID_PAT)
>   #define PPRO_FEATURES (CPUID_FP87 | CPUID_DE | CPUID_PSE | CPUID_TSC | \
>             CPUID_MSR | CPUID_MCE | CPUID_CX8 | CPUID_PGE | CPUID_CMOV | \
> -          CPUID_PAT | CPUID_FXSR | CPUID_MMX | CPUID_SSE | CPUID_SSE2 | \
> +          CPUID_MTRR | CPUID_FXSR | CPUID_MMX | CPUID_SSE | CPUID_SSE2 | \
>             CPUID_PAE | CPUID_SEP | CPUID_APIC)
>
>   #define TCG_FEATURES (CPUID_FP87 | CPUID_PSE | CPUID_TSC | CPUID_MSR | \
>