Re: [PATCH] KVM: VMX: Cache IA32_DEBUGCTL in memory

[Paolo Bonzini]

On 29/11/2017 09:51, Wanpeng Li wrote:
> 2017-11-29 16:48 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>:
>> On 29/11/2017 07:07, Wanpeng Li wrote:
>>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>>
>>> MSR_IA32_DEBUGCTLMSR is zeroed on VMEXIT, so it is saved/restored
>>> each time during world switch. Jim from Google pointed out that
>>> when running schbench in L2, vmx_vcpu_run will occupy 4% cpu time,
>>> and the 25% of vmx_vcpu_run cpu time is occupied by get_debugctlmsr().
>>> This patch caches the host IA32_DEBUGCTL MSR and saves/restores
>>> the host IA32_DEBUGCTL msr when guest/host switches to avoid to
>>> save/restore each time during world switch.
>>>
>>> Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
>>> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
>>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>
>> The update_debugctlmsr should stay in vmx_vcpu_run so that tracing
>> features work correctly.  However, the get_debugctlmsr indeed can be
> 
> The tracing can't run except vCPU is schedule out, so why
> update_debugctlmsr should stay in vmx_vcpu_run?

For example your patch is disabling BTS (branch trace store) after the
first vmexit, isn't it?

Thanks,

Paolo