linux-kernel - Re: [PATCH] KVM: SVM: Disable AVIC on SNP-enabled system without HvInUseWrAllowed feature

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Message-ID: <2d5f2f91-2c3a-4b0e-bacd-aeac6d4da724@oracle.com>
Date: Tue, 1 Oct 2024 12:04:47 +0100
From: Joao Martins <joao.m.martins@...cle.com>
To: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Cc: pbonzini@...hat.com, seanjc@...gle.com, david.kaplan@....com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: SVM: Disable AVIC on SNP-enabled system without
 HvInUseWrAllowed feature

On 30/09/2024 06:50, Suravee Suthikulpanit wrote:
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index dd4682857c12..921b6de80e24 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -448,6 +448,7 @@
>  #define X86_FEATURE_SME_COHERENT	(19*32+10) /* AMD hardware-enforced cache coherency */
>  #define X86_FEATURE_DEBUG_SWAP		(19*32+14) /* "debug_swap" AMD SEV-ES full debug state swap support */
>  #define X86_FEATURE_SVSM		(19*32+28) /* "svsm" SVSM present */
> +#define X86_FEATURE_HV_INUSE_WR_ALLOWED	(19*32+30) /* Write to in-use hypervisor-owned pages allowed */
>  
>  /* AMD-defined Extended Feature 2 EAX, CPUID level 0x80000021 (EAX), word 20 */
>  #define X86_FEATURE_NO_NESTED_DATA_BP	(20*32+ 0) /* No Nested Data Breakpoints */
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 4b74ea91f4e6..42f2caf17d6a 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -1199,6 +1199,12 @@ bool avic_hardware_setup(void)
>  		return false;
>  	}
>  
> +	if (cc_platform_has(CC_ATTR_HOST_SEV_SNP) &&
> +	    !boot_cpu_has(X86_FEATURE_HV_INUSE_WR_ALLOWED)) {
> +		pr_warn("AVIC disabled: missing HvInUseWrAllowed on SNP-enabled system");
> +		return false;
> +	}
> +

Wouldn't be better to make this is APICv inhibit to allow non-SNP guests to work
with AVIC?

	Joao