linux-kernel - Re: [PATCH v2] KVM: x86: Sync the states size with the XCR0/IA32

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Date:   Fri, 21 Jan 2022 15:41:53 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86: Sync the states size with the XCR0/IA32_XSS
 at any time

On Fri, Jan 21, 2022, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> XCR0 is reset to 1 by RESET but not INIT and IA32_XSS is zeroed by
> both RESET and INIT. The kvm_set_msr_common()'s handling of MSR_IA32_XSS
> also needs to update kvm_update_cpuid_runtime(). In the above cases, the
> size in bytes of the XSAVE area containing all states enabled by XCR0 or
> (XCRO | IA32_XSS) needs to be updated.
> 
> For simplicity and consistency, legacy helpers are used to write values

s/legacy/existing

"legacy" refers to something that is outdated/deprecated, which isn't what you
intend here.

> and call kvm_update_cpuid_runtime(), and it's not exactly a fast path.
> 
> Fixes: a554d207dc46 ("KVM: X86: Processor States following Reset or INIT")
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
> v1 -> v2 Changelog:
> - Strongly prefer that use the helpers to write the values; (Sean)
> - Postpone IA32_XSS test cases once non-zero values are supported; (Paolo)
> - User space may call SET_CPUID2 after kvm_vcpu_reset(init_event=false); (Paolo)
> 
>  arch/x86/kvm/x86.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 55518b7d3b96..22d4b1d15e94 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11256,7 +11256,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> 
>  		vcpu->arch.msr_misc_features_enables = 0;
> 
> -		vcpu->arch.xcr0 = XFEATURE_MASK_FP;
> +		__kvm_set_xcr(vcpu, 0, XFEATURE_MASK_FP);
>  	}
> 
>  	/* All GPRs except RDX (handled below) are zeroed on RESET/INIT. */
> @@ -11273,7 +11273,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	cpuid_0x1 = kvm_find_cpuid_entry(vcpu, 1, 0);
>  	kvm_rdx_write(vcpu, cpuid_0x1 ? cpuid_0x1->eax : 0x600);
> 
> -	vcpu->arch.ia32_xss = 0;
> +	__kvm_set_msr(vcpu, MSR_IA32_XSS, 0, true);

This relies on a prep patch to invoke kvm_update_cpuid_runtime() in kvm_set_msr_common()
for MSR_IA32_XSS, no?

>  	static_call(kvm_x86_vcpu_reset)(vcpu, init_event);
> 
> -- 
> 2.33.1
> 
>