linux-kernel - Re: [PATCH v3 9/9] KVM: x86: Add XSAVE Support for Architectural LBRs

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Date:   Wed, 3 Mar 2021 10:03:14 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, wei.w.wang@...el.com,
        Borislav Petkov <bp@...en8.de>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 9/9] KVM: x86: Add XSAVE Support for Architectural LBRs

On Wed, Mar 03, 2021, Like Xu wrote:
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 034708a3df20..ec4593e0ee6d 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7268,6 +7268,8 @@ static __init void vmx_set_cpu_caps(void)
>  	supported_xss = 0;
>  	if (!cpu_has_vmx_xsaves())
>  		kvm_cpu_cap_clear(X86_FEATURE_XSAVES);
> +	else if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR))
> +		supported_xss |= XFEATURE_MASK_LBR;
>  
>  	/* CPUID 0x80000001 */
>  	if (!cpu_has_vmx_rdtscp())
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d773836ceb7a..bca2e318ff24 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10433,6 +10433,8 @@ int kvm_arch_hardware_setup(void *opaque)
>  
>  	if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
>  		supported_xss = 0;
> +	else
> +		supported_xss &= host_xss;

Not your fault by any means, but I would prefer to have matching logic for XSS
and XCR0.  The existing clearing of supported_xss here is pointless.  E.g. I'd
prefer something like the following, though Paolo may have a different opinion.

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 6d7e760fdfa0..c781034463e5 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7244,12 +7244,15 @@ static __init void vmx_set_cpu_caps(void)
                kvm_cpu_cap_clear(X86_FEATURE_INVPCID);
        if (vmx_pt_mode_is_host_guest())
                kvm_cpu_cap_check_and_set(X86_FEATURE_INTEL_PT);
+       if (!cpu_has_vmx_arch_lbr()) {
+               kvm_cpu_cap_clear(X86_FEATURE_ARCH_LBR);
+               supported_xss &= ~XFEATURE_MASK_LBR;
+       }

        if (vmx_umip_emulated())
                kvm_cpu_cap_set(X86_FEATURE_UMIP);

        /* CPUID 0xD.1 */
-       supported_xss = 0;
        if (!cpu_has_vmx_xsaves())
                kvm_cpu_cap_clear(X86_FEATURE_XSAVES);

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7b0adebec1ef..5f9eb1f5b840 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -205,6 +205,8 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
                                | XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
                                | XFEATURE_MASK_PKRU)

+#define KVM_SUPPORTED_XSS      XFEATURE_MASK_LBR
+
 u64 __read_mostly host_efer;
 EXPORT_SYMBOL_GPL(host_efer);

@@ -8037,6 +8039,11 @@ int kvm_arch_init(void *opaque)
                supported_xcr0 = host_xcr0 & KVM_SUPPORTED_XCR0;
        }

+       if (boot_cpu_has(X86_FEATURE_XSAVES))
+               rdmsrl(MSR_IA32_XSS, host_xss);
+               supported_xss = host_xss & KVM_SUPPORTED_XSS;
+       }
+
        if (pi_inject_timer == -1)
                pi_inject_timer = housekeeping_enabled(HK_FLAG_TIMER);
 #ifdef CONFIG_X86_64
@@ -10412,9 +10419,6 @@ int kvm_arch_hardware_setup(void *opaque)

        rdmsrl_safe(MSR_EFER, &host_efer);

-       if (boot_cpu_has(X86_FEATURE_XSAVES))
-               rdmsrl(MSR_IA32_XSS, host_xss);
-
        r = ops->hardware_setup();
        if (r != 0)
                return r;
@@ -10422,9 +10426,6 @@ int kvm_arch_hardware_setup(void *opaque)
        memcpy(&kvm_x86_ops, ops->runtime_ops, sizeof(kvm_x86_ops));
        kvm_ops_static_call_update();

-       if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
-               supported_xss = 0;
-
 #define __kvm_cpu_cap_has(UNUSED_, f) kvm_cpu_cap_has(f)
        cr4_reserved_bits = __cr4_reserved_bits(__kvm_cpu_cap_has, UNUSED_);
 #undef __kvm_cpu_cap_has