Re: [PATCH 39/61] KVM: SVM: Convert feature updates from CPUID to KVM cpu caps

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 39/61] KVM: SVM: Convert feature updates from CPUID to KVM cpu caps
Date: Mon, 24 Feb 2020 22:33:34 +0100	[thread overview]
Message-ID: <87eeujoekx.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20200201185218.24473-40-sean.j.christopherson@intel.com>

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> Use the recently introduced KVM CPU caps to propagate SVM-only (kernel)
> settings to supported CPUID flags.
>
> Note, setting a flag based on a *different* feature is effectively
> emulation, and so must be done at runtime via ->set_supported_cpuid().
>
> Opportunistically add a technically unnecessary break and fix an
> indentation issue in svm_set_supported_cpuid().
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  arch/x86/kvm/svm.c | 40 +++++++++++++++++++++++-----------------
>  1 file changed, 23 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 630520f8adfa..f98a192459f7 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1350,6 +1350,25 @@ static __init void svm_adjust_mmio_mask(void)
>  	kvm_mmu_set_mmio_spte_mask(mask, mask, PT_WRITABLE_MASK | PT_USER_MASK);
>  }
>  

Can we probably add the comment about what can be done here and what
needs to go to svm_set_supported_cpuid()? (The one about 'emulation'
from your commit message would do).

> +static __init void svm_set_cpu_caps(void)
> +{
> +	/* CPUID 0x1 */
> +	if (avic)
> +		kvm_cpu_cap_clear(X86_FEATURE_X2APIC);
> +
> +	/* CPUID 0x80000001 */
> +	if (nested)
> +		kvm_cpu_cap_set(X86_FEATURE_SVM);
> +
> +	/* CPUID 0x8000000A */
> +	/* Support next_rip if host supports it */
> +	if (boot_cpu_has(X86_FEATURE_NRIPS))
> +		kvm_cpu_cap_set(X86_FEATURE_NRIPS);

Unrelated to your patch but the way we handle 'nrips' is a bit weird: we
can disable it with 'nrips' module parameter but L1 hypervisor will get
it unconditionally.

Also, what about all the rest of 0x8000000A.EDX features? Nested SVM
would appreciate some love... 

> +
> +	if (npt_enabled)
> +		kvm_cpu_cap_set(X86_FEATURE_NPT);
> +}
> +
>  static __init int svm_hardware_setup(void)
>  {
>  	int cpu;
> @@ -1462,6 +1481,8 @@ static __init int svm_hardware_setup(void)
>  			pr_info("Virtual GIF supported\n");
>  	}
>  
> +	svm_set_cpu_caps();
> +
>  	return 0;
>  
>  err:
> @@ -6033,17 +6054,9 @@ static void svm_cpuid_update(struct kvm_vcpu *vcpu)
>  static void svm_set_supported_cpuid(struct kvm_cpuid_entry2 *entry)
>  {
>  	switch (entry->function) {
> -	case 0x1:
> -		if (avic)
> -			cpuid_entry_clear(entry, X86_FEATURE_X2APIC);
> -		break;
> -	case 0x80000001:
> -		if (nested)
> -			cpuid_entry_set(entry, X86_FEATURE_SVM);
> -		break;
>  	case 0x80000008:
>  		if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) ||
> -		     boot_cpu_has(X86_FEATURE_AMD_SSBD))
> +		    boot_cpu_has(X86_FEATURE_AMD_SSBD))
>  			cpuid_entry_set(entry, X86_FEATURE_VIRT_SSBD);
>  		break;
>  	case 0x8000000A:
> @@ -6053,14 +6066,7 @@ static void svm_set_supported_cpuid(struct kvm_cpuid_entry2 *entry)
>  		entry->ecx = 0; /* Reserved */
>  		entry->edx = 0; /* Per default do not support any
>  				   additional features */
> -
> -		/* Support next_rip if host supports it */
> -		if (boot_cpu_has(X86_FEATURE_NRIPS))
> -			cpuid_entry_set(entry, X86_FEATURE_NRIPS);
> -
> -		/* Support NPT for the guest if enabled */
> -		if (npt_enabled)
> -			cpuid_entry_set(entry, X86_FEATURE_NPT);
> +		break;
>  	}
>  }

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly