linux-kernel - Re: [PATCH v14 05/22] KVM: x86: Initialize kvm_caps.supported

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Message-ID: <9197eea9-e8fb-49e7-a74d-ee4e0401074f@intel.com>
Date: Wed, 10 Sep 2025 17:36:25 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Chao Gao <chao.gao@...el.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: acme@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
 hpa@...or.com, john.allen@....com, mingo@...nel.org, mingo@...hat.com,
 minipli@...ecurity.net, mlevitsk@...hat.com, namhyung@...nel.org,
 pbonzini@...hat.com, prsampat@....com, rick.p.edgecombe@...el.com,
 seanjc@...gle.com, shuah@...nel.org, tglx@...utronix.de,
 weijiang.yang@...el.com, x86@...nel.org, xin@...or.com
Subject: Re: [PATCH v14 05/22] KVM: x86: Initialize kvm_caps.supported_xss

On 9/9/2025 5:39 PM, Chao Gao wrote:
> From: Yang Weijiang <weijiang.yang@...el.com>
> 
> Set original kvm_caps.supported_xss to (host_xss & KVM_SUPPORTED_XSS) if
> XSAVES is supported. host_xss contains the host supported xstate feature
> bits for thread FPU context switch, KVM_SUPPORTED_XSS includes all KVM
> enabled XSS feature bits, the resulting value represents the supervisor
> xstates that are available to guest and are backed by host FPU framework
> for swapping {guest,host} XSAVE-managed registers/MSRs.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> Reviewed-by: Chao Gao <chao.gao@...el.com>
> Tested-by: Mathias Krause <minipli@...ecurity.net>
> Tested-by: John Allen <john.allen@....com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>
> Signed-off-by: Chao Gao <chao.gao@...el.com>
> ---
>   arch/x86/kvm/x86.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index bbae3bf405c7..c15e8c00dc7d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -220,6 +220,8 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
>   				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
>   				| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)
>   
> +#define KVM_SUPPORTED_XSS     0
> +

(related to my comment on previous patch)

It seems better to move the comment about Intel PT (IA32_XSS[bit 8]) in 
kvm_set_msr_common() to above KVM_SUPPORTED_XSS.

>   bool __read_mostly allow_smaller_maxphyaddr = 0;
>   EXPORT_SYMBOL_GPL(allow_smaller_maxphyaddr);
>   
> @@ -9789,14 +9791,17 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
>   		kvm_host.xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
>   		kvm_caps.supported_xcr0 = kvm_host.xcr0 & KVM_SUPPORTED_XCR0;
>   	}
> +
> +	if (boot_cpu_has(X86_FEATURE_XSAVES)) {
> +		rdmsrq(MSR_IA32_XSS, kvm_host.xss);
> +		kvm_caps.supported_xss = kvm_host.xss & KVM_SUPPORTED_XSS;
> +	}
> +
>   	kvm_caps.supported_quirks = KVM_X86_VALID_QUIRKS;
>   	kvm_caps.inapplicable_quirks = KVM_X86_CONDITIONAL_QUIRKS;
>   
>   	rdmsrq_safe(MSR_EFER, &kvm_host.efer);
>   
> -	if (boot_cpu_has(X86_FEATURE_XSAVES))
> -		rdmsrq(MSR_IA32_XSS, kvm_host.xss);
> -
>   	kvm_init_pmu_capability(ops->pmu_ops);
>   
>   	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))