linux-kernel - Re: [PATCH] KVM: vmx/pmu: Add VMCS fields check before exposing LBR

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Date:   Wed, 3 Feb 2021 10:42:22 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: vmx/pmu: Add VMCS fields check before exposing
 LBR_FMT

On 03/02/21 07:50, Like Xu wrote:
> Before KVM exposes guest LBR_FMT perf capabilities, it needs to check
> whether VMCS has GUEST_IA32_DEBUGCTL guest status field and vmx switch
> support on IA32_DEBUGCTL MSR (including VM_EXIT_SAVE_DEBUG_CONTROLS
> and VM_ENTRY_LOAD_DEBUG_CONTROLS). It helps nested LBR enablement.
> 
> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> ---
>   arch/x86/kvm/vmx/capabilities.h | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
> index d1d77985e889..ac3af06953a8 100644
> --- a/arch/x86/kvm/vmx/capabilities.h
> +++ b/arch/x86/kvm/vmx/capabilities.h
> @@ -378,6 +378,12 @@ static inline bool vmx_pt_mode_is_host_guest(void)
>   	return pt_mode == PT_MODE_HOST_GUEST;
>   }
>   
> +static inline bool cpu_has_vmx_lbr(void)
> +{
> +	return (vmcs_config.vmexit_ctrl & VM_EXIT_SAVE_DEBUG_CONTROLS) &&
> +		(vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_DEBUG_CONTROLS);
> +}
> +
>   static inline u64 vmx_get_perf_capabilities(void)
>   {
>   	u64 perf_cap = 0;
> @@ -385,7 +391,8 @@ static inline u64 vmx_get_perf_capabilities(void)
>   	if (boot_cpu_has(X86_FEATURE_PDCM))
>   		rdmsrl(MSR_IA32_PERF_CAPABILITIES, perf_cap);
>   
> -	perf_cap &= PMU_CAP_LBR_FMT;
> +	if (cpu_has_vmx_lbr())
> +		perf_cap &= PMU_CAP_LBR_FMT;

This is incorrect in the case where cpu_has_vmx_lbr() is false.  You 
would need something like

  	u64 perf_cap = 0;
	u64 host_perf_cap = 0;

  	if (boot_cpu_has(X86_FEATURE_PDCM))
		rdmsrl(MSR_IA32_PERF_CAPABILITIES, host_perf_cap);

	if (cpu_has_vmx_lbr())
		perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;

However, KVM won't run without VM_ENTRY_LOAD_DEBUG_CONTROLS and 
VM_EXIT_SAVE_DEBUG_CONTROLS (see setup_vmcs_config), so this change is 
not needed either.

Paolo