Re: [PATCH 0/3] KVM: x86: guest interface for SEV live migration

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 20, 2021, Paolo Bonzini wrote:
> From 547d4d4edcd05fdfac6ce650d65db1d42bcd2807 Mon Sep 17 00:00:00 2001
> From: Paolo Bonzini <pbonzini@redhat.com>
> Date: Tue, 20 Apr 2021 05:49:11 -0400
> Subject: [PATCH 1/3] KVM: SEV: mask CPUID[0x8000001F].eax according to
>  supported features

Your mailer is obviously a bit wonky, took me a while to find this patch :-)
 
> Do not return the SEV-ES bit from KVM_GET_SUPPORTED_CPUID unless
> the corresponding module parameter is 1, and clear the memory encryption
> leaf completely if SEV is disabled.

Impeccable timing, I was planning on refreshing my SEV cleanup series[*] today.
There's going to be an annoying conflict with the svm_set_cpu_caps() change
(see below), any objecting to folding your unintentional feedback into my series?

[*] https://lkml.kernel.org/r/20210306015905.186698-1-seanjc@google.com

> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/cpuid.c   | 5 ++++-
>  arch/x86/kvm/cpuid.h   | 1 +
>  arch/x86/kvm/svm/svm.c | 7 +++++++
>  3 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 2ae061586677..d791d1f093ab 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -944,8 +944,11 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		break;
>  	/* Support memory encryption cpuid if host supports it */
>  	case 0x8000001F:
> -		if (!boot_cpu_has(X86_FEATURE_SEV))
> +		if (!kvm_cpu_cap_has(X86_FEATURE_SEV)) {
>  			entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
> +			break;
> +		}
> +		cpuid_entry_override(entry, CPUID_8000_001F_EAX);

I find this easier to read:

		if (!kvm_cpu_cap_has(X86_FEATURE_SEV))
			entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
		else
			cpuid_entry_override(entry, CPUID_8000_001F_EAX);

>  		break;
>  	/*Add support for Centaur's CPUID instruction*/
>  	case 0xC0000000:
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index 888e88b42e8d..e873a60a4830 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -99,6 +99,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
>  	[CPUID_7_EDX]         = {         7, 0, CPUID_EDX},
>  	[CPUID_7_1_EAX]       = {         7, 1, CPUID_EAX},
>  	[CPUID_12_EAX]        = {0x00000012, 0, CPUID_EAX},
> +	[CPUID_8000_001F_EAX] = {0x8000001F, 0, CPUID_EAX},
>  };
>  
>  /*
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index cd8c333ed2dc..acdb8457289e 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -923,6 +923,13 @@ static __init void svm_set_cpu_caps(void)
>  	if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) ||
>  	    boot_cpu_has(X86_FEATURE_AMD_SSBD))
>  		kvm_cpu_cap_set(X86_FEATURE_VIRT_SSBD);
> +
> +	/* CPUID 0x8000001F */
> +	if (sev) {
> +		kvm_cpu_cap_set(X86_FEATURE_SEV);
> +		if (sev_es)
> +			kvm_cpu_cap_set(X86_FEATURE_SEV_ES);

Gah, I completely spaced on the module params in my series, which is more
problematic than normal because it also moves "sev" and "sev_es" to sev.c.  The
easy solution is to add sev_set_cpu_caps().

On the other, this misses SME_COHERENT.  I also think it makes sense to call
kvm_cpu_cap_mask() for the leaf, even if it's just to crush KVM's caps to zero.
However, because of SME_COHERENT and other potential bits in the future, I think
I prefer starting with the bits carried over from boot_cpu_data.  E.g.

	kvm_cpu_cap_mask(CPUID_8000_001F_EAX,
		0 /* SME */ | F(SEV) | 0 /* VM_PAGE_FLUSH */ | F(SEV_ES) |
		F(SME_COHERENT));

and (with renamed module params):

	if (sev_enabled)
		kvm_cpu_cap_clear(X86_FEATURE_SEV);
	if (sev_es_enabled)
		kvm_cpu_cap_clear(X86_FEATURE_SEV_ES);

> +	}
>  }
>  
>  static __init int svm_hardware_setup(void)