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Message-ID: <ZmoPLOx8sujVsGIS@google.com>
Date: Wed, 12 Jun 2024 14:12:12 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xin Li <xin3.li@...el.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
pbonzini@...hat.com, corbet@....net, tglx@...utronix.de, mingo@...hat.com,
bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
shuah@...nel.org, vkuznets@...hat.com, peterz@...radead.org,
ravi.v.shankar@...el.com, xin@...or.com
Subject: Re: [PATCH v2 04/25] KVM: x86: Mark CR4.FRED as not reserved
On Wed, Feb 07, 2024, Xin Li wrote:
> The CR4.FRED bit, i.e., CR4[32], is no longer a reserved bit when a guest
> enumerates FRED, otherwise it is still a reserved bit.
This isn't quite correct, as __cr4_reserved_bits() is used with kvm_cpu_caps too,
i.e. to compute CR4 bits that are reserved from the host's perspective. And that
matters, because if this check was done _only_ on guest CPUID, then KVM would
allow CR4.FRED=1 before all of KVM support is in place.
> Signed-off-by: Xin Li <xin3.li@...el.com>
> Tested-by: Shan Kang <shan.kang@...el.com>
> ---
> arch/x86/include/asm/kvm_host.h | 2 +-
> arch/x86/kvm/x86.h | 2 ++
> 2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b5b2d0fde579..0d88873eba63 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -134,7 +134,7 @@
> | X86_CR4_OSXSAVE | X86_CR4_SMEP | X86_CR4_FSGSBASE \
> | X86_CR4_OSXMMEXCPT | X86_CR4_LA57 | X86_CR4_VMXE \
> | X86_CR4_SMAP | X86_CR4_PKE | X86_CR4_UMIP \
> - | X86_CR4_LAM_SUP))
> + | X86_CR4_LAM_SUP | X86_CR4_FRED))
>
> #define CR8_RESERVED_BITS (~(unsigned long)X86_CR8_TPR)
>
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 2f7e19166658..9a52016ebf5a 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -532,6 +532,8 @@ bool kvm_msr_allowed(struct kvm_vcpu *vcpu, u32 index, u32 type);
> __reserved_bits |= X86_CR4_PCIDE; \
> if (!__cpu_has(__c, X86_FEATURE_LAM)) \
> __reserved_bits |= X86_CR4_LAM_SUP; \
> + if (!__cpu_has(__c, X86_FEATURE_FRED)) \
> + __reserved_bits |= X86_CR4_FRED; \
> __reserved_bits; \
> })
>
> --
> 2.43.0
>
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