linux-kernel - Re: [PATCH v11 5/9] KVM: X86: Refresh CPUID once guest XSS MSR changes

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Date:   Thu, 23 Apr 2020 10:34:50 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, jmattson@...gle.com,
        yu.c.zhang@...ux.intel.com
Subject: Re: [PATCH v11 5/9] KVM: X86: Refresh CPUID once guest XSS MSR
 changes

On Thu, Mar 26, 2020 at 04:18:42PM +0800, Yang Weijiang wrote:
> CPUID(0xd, 1) reports the current required storage size of
> XCR0 | XSS, when guest updates the XSS, it's necessary to update
> the CPUID leaf, otherwise guest will fetch old state size, and
> results to some WARN traces during guest running.
> 
> Co-developed-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> Signed-off-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  1 +
>  arch/x86/kvm/cpuid.c            | 21 ++++++++++++++++++---
>  arch/x86/kvm/x86.c              |  9 +++++++--
>  3 files changed, 26 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 24c90ea5ddbd..2c944ad99692 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -650,6 +650,7 @@ struct kvm_vcpu_arch {
>  
>  	u64 xcr0;
>  	u64 guest_supported_xcr0;
> +	u64 guest_supported_xss;
>  	u32 guest_xstate_size;
>  
>  	struct kvm_pio_request pio;
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 78d461be2102..25e9a11291b3 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -95,9 +95,24 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
>  	}
>  
>  	best = kvm_find_cpuid_entry(vcpu, 0xD, 1);
> -	if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
> -		     cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
> -		best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> +	if (best) {
> +		if (best->eax & (F(XSAVES) | F(XSAVEC))) {

Please use cpuid_entry_has() to preserve the automagic register lookup and
compile-time assertions that are provided.  E.g. I don't know off the top
of my whether %eax is the correct register, and I don't want to know :-).

> +			u64 xstate = vcpu->arch.xcr0 | vcpu->arch.ia32_xss;
> +
> +			best->ebx = xstate_required_size(xstate, true);
> +		}
> +
> +		if (best->eax & F(XSAVES)) {

Same thing here.

> +			vcpu->arch.guest_supported_xss =
> +			(best->ecx | ((u64)best->edx << 32)) & supported_xss;

The indentation is funky, I'm guessing you're trying to squeak in less than
80 chars.  Maybe this?

		if (!cpuid_entry_has(best, X86_FEATURE_XSAVES)) {
			best->ecx = 0;
			best->edx = 0;
		}

		 vcpu->arch.guest_supported_xss =
			(((u64)best->edx << 32) | best->ecx) & supported_xss;

Nit: my preference is to have the high half first, x86 is little endian
(the xcr0 code is "wrong" :-D).  For me, this also makes it more obvious
that the effective size is a u64.

> +		} else {
> +			best->ecx = 0;
> +			best->edx = 0;
> +			vcpu->arch.guest_supported_xss = 0;
> +		}
> +	} else {
> +		vcpu->arch.guest_supported_xss = 0;
> +	}
>  
>  	/*
>  	 * The existing code assumes virtual address is 48-bit or 57-bit in the
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 90acdbbb8a5a..51ecb496d47d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2836,9 +2836,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		 * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
>  		 * XSAVES/XRSTORS to save/restore PT MSRs.
>  		 */
> -		if (data & ~supported_xss)
> +		if (data & ~vcpu->arch.guest_supported_xss)
>  			return 1;
> -		vcpu->arch.ia32_xss = data;
> +		if (vcpu->arch.ia32_xss != data) {
> +			vcpu->arch.ia32_xss = data;
> +			kvm_update_cpuid(vcpu);
> +		}
>  		break;
>  	case MSR_SMI_COUNT:
>  		if (!msr_info->host_initiated)
> @@ -9635,6 +9638,8 @@ int kvm_arch_hardware_setup(void)
>  
>  	if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
>  		supported_xss = 0;
> +	else
> +		supported_xss = host_xss & KVM_SUPPORTED_XSS;

Silly nit: I'd prefer to invert the check, e.g.

	if (kvm_cpu_cap_has(X86_FEATURE_XSAVES))
		supported_xss = host_xss & KVM_SUPPORTED_XSS;
	else
		supported_xss = 0;

>  
>  	cr4_reserved_bits = kvm_host_cr4_reserved_bits(&boot_cpu_data);
>  
> -- 
> 2.17.2
>