Re: [PATCH v2] KVM: VMX: Cache IA32_DEBUGCTL in memory

[Jim Mattson]

Thanks for doing this!

Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>

On Wed, Nov 29, 2017 at 1:31 AM, Wanpeng Li <kernellwp@xxxxxxxxx> 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: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> v1 ->  v2:
>  * rename to host_debugctlmsr and place it in struct vcpu_struct
>  * update_debugctlmsr stay in vmx_vcpu_run
>
>  arch/x86/kvm/vmx.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 8c7e816..d293c29 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -658,6 +658,8 @@ struct vcpu_vmx {
>
>         u32 host_pkru;
>
> +       unsigned long host_debugctlmsr;
> +
>         /*
>          * Only bits masked by msr_ia32_feature_control_valid_bits can be set in
>          * msr_ia32_feature_control. FEATURE_CONTROL_LOCKED is always included
> @@ -2326,6 +2328,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>
>         vmx_vcpu_pi_load(vcpu, cpu);
>         vmx->host_pkru = read_pkru();
> +       vmx->host_debugctlmsr = get_debugctlmsr();
>  }
>
>  static void vmx_vcpu_pi_put(struct kvm_vcpu *vcpu)
> @@ -9346,7 +9349,7 @@ static void vmx_arm_hv_timer(struct kvm_vcpu *vcpu)
>  static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  {
>         struct vcpu_vmx *vmx = to_vmx(vcpu);
> -       unsigned long debugctlmsr, cr3, cr4;
> +       unsigned long cr3, cr4;
>
>         /* Record the guest's net vcpu time for enforced NMI injections. */
>         if (unlikely(!enable_vnmi &&
> @@ -9399,7 +9402,6 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>                 __write_pkru(vcpu->arch.pkru);
>
>         atomic_switch_perf_msrs(vmx);
> -       debugctlmsr = get_debugctlmsr();
>
>         vmx_arm_hv_timer(vcpu);
>
> @@ -9510,8 +9512,8 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>               );
>
>         /* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
> -       if (debugctlmsr)
> -               update_debugctlmsr(debugctlmsr);
> +       if (vmx->host_debugctlmsr)
> +               update_debugctlmsr(vmx->host_debugctlmsr);
>
>  #ifndef CONFIG_X86_64
>         /*
> --
> 2.7.4
>