Re: [PATCH] KVM: x86: Update the states size cpuid even if XCR0/IA32_XSS is reset

[Like Xu <like.xu.linux@gmail.com>]

On 19/1/2022 2:30 am, Sean Christopherson wrote:
> On Mon, Jan 17, 2022, Like Xu wrote:
>> From: Like Xu <likexu@tencent.com>
>>
>> XCR0 is reset to 1 by RESET but not INIT and IA32_XSS is zeroed by
>> both RESET and INIT. In both cases, the size in bytes of the XSAVE
>> area containing all states enabled by XCR0 or (XCRO | IA32_XSS)
>> needs to be updated.
>>
>> Fixes: a554d207dc46 ("KVM: X86: Processor States following Reset or INIT")
>> Signed-off-by: Like Xu <likexu@tencent.com>
>> ---
>>   arch/x86/kvm/x86.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 76b4803dd3bd..5748a57e1cb7 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -11134,6 +11134,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>>   	struct kvm_cpuid_entry2 *cpuid_0x1;
>>   	unsigned long old_cr0 = kvm_read_cr0(vcpu);
>>   	unsigned long new_cr0;
>> +	bool need_update_cpuid = false;
>>   
>>   	/*
>>   	 * Several of the "set" flows, e.g. ->set_cr0(), read other registers
>> @@ -11199,6 +11200,8 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>>   
>>   		vcpu->arch.msr_misc_features_enables = 0;
>>   
>> +		if (vcpu->arch.xcr0 != XFEATURE_MASK_FP)
>> +			need_update_cpuid = true;
>>   		vcpu->arch.xcr0 = XFEATURE_MASK_FP;
>>   	}
>>   
>> @@ -11216,6 +11219,8 @@ 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);
>>   
>> +	if (vcpu->arch.ia32_xss)
>> +		need_update_cpuid = true;
> 
> This means that kvm_set_msr_common()'s handling of MSR_IA32_XSS also needs to
> update kvm_update_cpuid_runtime().  And then for bnoth XCR0 and XSS, I would very
> strongly prefer that use the helpers to write the values and let the helpers call

Looks good to me and let me apply it in the next version.

> kvm_update_cpuid_runtime().  Yes, that will mean kvm_update_cpuid_runtime() may be
> called multiple times during INIT, but that's already true (CR4), and this isn't
> exactly a fast path.

An undisclosed lazy mechanism is under analyzed for performance gains.

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