linux-kernel - Re: [PATCH v3 2/6] KVM: x86: Introduce CPUID_8000_0007

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Date:   Tue, 20 Sep 2022 22:52:53 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/6] KVM: x86: Introduce CPUID_8000_0007_EDX
 'scattered' leaf

On Fri, Sep 16, 2022, Vitaly Kuznetsov wrote:
> CPUID_8000_0007_EDX may come handy when X86_FEATURE_CONSTANT_TSC
> needs to be checked.
> 
> No functional change intended.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/cpuid.c         | 4 ++++
>  arch/x86/kvm/reverse_cpuid.h | 9 ++++++++-
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 75dcf7a72605..f68b14053c9b 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -669,6 +669,10 @@ void kvm_set_cpu_caps(void)
>  	if (!tdp_enabled && IS_ENABLED(CONFIG_X86_64))
>  		kvm_cpu_cap_set(X86_FEATURE_GBPAGES);
>  
> +	kvm_cpu_cap_init_scattered(CPUID_8000_0007_EDX,
> +		SF(CONSTANT_TSC)
> +	);

The scattered leaf needs to be used in __do_cpuid_func(), e.g.

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index ffdc28684cb7..c91f23bb3605 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1137,8 +1137,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
                /* L2 cache and TLB: pass through host info. */
                break;
        case 0x80000007: /* Advanced power management */
-               /* invariant TSC is CPUID.80000007H:EDX[8] */
-               entry->edx &= (1 << 8);
+               cpuid_entry_override(entry, CPUID_8000_0007_EDX);
+
                /* mask against host */
                entry->edx &= boot_cpu_data.x86_power;
                entry->eax = entry->ebx = entry->ecx = 0;