linux-kernel - Re: [PATCH v13 03/21] KVM: x86: Refresh CPUID on write to guest MSR_IA32

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Message-ID: <aLGDk39uByD8pPMW@intel.com>
Date: Fri, 29 Aug 2025 18:40:19 +0800
From: Chao Gao <chao.gao@...el.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>, <hpa@...or.com>, <john.allen@....com>,
	<mingo@...hat.com>, <minipli@...ecurity.net>, <mlevitsk@...hat.com>,
	<pbonzini@...hat.com>, <rick.p.edgecombe@...el.com>, <seanjc@...gle.com>,
	<tglx@...utronix.de>, <weijiang.yang@...el.com>, <x86@...nel.org>,
	<xin@...or.com>, Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v13 03/21] KVM: x86: Refresh CPUID on write to guest
 MSR_IA32_XSS

On Fri, Aug 29, 2025 at 02:47:37PM +0800, Xiaoyao Li wrote:
>On 8/21/2025 9:30 PM, Chao Gao wrote:
>> From: Yang Weijiang <weijiang.yang@...el.com>
>> 
>> Update CPUID.(EAX=0DH,ECX=1).EBX to reflect current required xstate size
>> due to XSS MSR modification.
>> CPUID(EAX=0DH,ECX=1).EBX reports the required storage size of all enabled
>> xstate features in (XCR0 | IA32_XSS). The CPUID value can be used by guest
>> before allocate sufficient xsave buffer.
>> 
>> Note, KVM does not yet support any XSS based features, i.e. supported_xss
>> is guaranteed to be zero at this time.
>> 
>> Opportunistically return KVM_MSR_RET_UNSUPPORTED if guest CPUID doesn't
>> enumerate it. Since KVM_MSR_RET_UNSUPPORTED takes care of host_initiated
>> cases, drop the host_initiated check.
>
>It looks most of this patch is introducing
>
>  vcpu->arch.guest_supported_xss
>
>and use it to guard the wrmsr of MSR_IA32_XSS instead of KVM global
>kvm_caps.supported_xss.
>
>So it's better to split it into two patches?

Yes. it makes sense. will do.