linux-kernel - Re: [PATCH] x86, kvm, vmx: Always use LOAD_IA32

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Date:	Mon, 10 Nov 2014 11:31:57 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>
Cc:	kvm list <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Gleb Natapov <gleb@...nel.org>
Subject: Re: [PATCH] x86, kvm, vmx: Always use LOAD_IA32_EFER if available

On Sun, Nov 9, 2014 at 8:33 PM, Wanpeng Li <wanpeng.li@...ux.intel.com> wrote:
> Hi Andy,
> On Fri, Nov 07, 2014 at 06:25:18PM -0800, Andy Lutomirski wrote:
>>At least on Sandy Bridge, letting the CPU switch IA32_EFER is much
>>faster than switching it manually.
>>
>>I benchmarked this using the vmexit kvm-unit-test (single run, but
>>GOAL multiplied by 5 to do more iterations):
>>
>>Test                                  Before      After    Change
>>cpuid                                   2000       1932    -3.40%
>>vmcall                                  1914       1817    -5.07%
>>mov_from_cr8                              13         13     0.00%
>>mov_to_cr8                                19         19     0.00%
>>inl_from_pmtimer                       19164      10619   -44.59%
>>inl_from_qemu                          15662      10302   -34.22%
>
> What's the difference of IA32_EFER between guest and host in your config?
>
> IIUC,
> - NX is not consistent
>   IA32_EFER will be auto load w/ and w/o the patch.
> - SCE is not consistent
>   IA32_EFER will be switched through wrmsr(urn) w/o the patch, and auto load
>   w/ the patch.

This is with kvm-unit-test as is, so NX is consistent but SCE is different.

--Andy
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