linux-kernel - Re: [PATCH 00/61] KVM: x86: Introduce KVM cpu caps

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Date:   Sat, 29 Feb 2020 10:32:19 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/61] KVM: x86: Introduce KVM cpu caps

On Tue, Feb 25, 2020 at 04:18:38PM +0100, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@...el.com> writes:
> 
> >
> >   7. Profit!
> 
> Would it be better or worse if we eliminate set_supported_cpuid() hook
> completely by doing an ugly hack like (completely untested):
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index a2a091d328c6..5ad291d48e1b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1145,8 +1145,6 @@ struct kvm_x86_ops {
>  
>         void (*set_tdp_cr3)(struct kvm_vcpu *vcpu, unsigned long cr3);
>  
> -       void (*set_supported_cpuid)(struct kvm_cpuid_entry2 *entry);
> -
>         bool (*has_wbinvd_exit)(void);
>  
>         u64 (*read_l1_tsc_offset)(struct kvm_vcpu *vcpu);
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index e8beb1e542a8..88431fc02797 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -749,6 +749,16 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		cpuid_entry_override(entry, CPUID_8000_0008_EBX);
>  		break;
>  	}
> +	case 0x8000000A:
> +		if (boot_cpu_has(X86_FEATURE_SVM)) {
> +			entry->eax = 1; /* SVM revision 1 */
> +			entry->ebx = 8; /* Lets support 8 ASIDs in case we add proper
> +					   ASID emulation to nested SVM */
> +			entry->ecx = 0; /* Reserved */
> +			entry->edx = 0; /* Per default do not support any
> +					   additional features */

Lucky thing that you suggested this change, patch ("KVM: SVM: Convert
feature updates from CPUID to KVM cpu caps") was buggy in that clearing
entry->edx here would wipe out all X86_FEATURE_NRIPS and X86_FEATURE_NPT.
Only noticed it when moving this code.