linux-kernel - Re: [PATCH v15 09/41] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs

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Message-ID: <34eeea73-0f38-4471-b06b-aa6d598851ee@intel.com>
Date: Tue, 16 Sep 2025 14:51:56 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Tom Lendacky <thomas.lendacky@....com>,
 Mathias Krause <minipli@...ecurity.net>, John Allen <john.allen@....com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>, Chao Gao <chao.gao@...el.com>,
 Maxim Levitsky <mlevitsk@...hat.com>, Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v15 09/41] KVM: x86: Load guest FPU state when access
 XSAVE-managed MSRs

On 9/13/2025 7:22 AM, Sean Christopherson wrote:
> Load the guest's FPU state if userspace is accessing MSRs whose values
> are managed by XSAVES. Introduce two helpers, kvm_{get,set}_xstate_msr(),
> to facilitate access to such kind of MSRs.
> 
> If MSRs supported in kvm_caps.supported_xss are passed through to guest,
> the guest MSRs are swapped with host's before vCPU exits to userspace and
> after it reenters kernel before next VM-entry.
> 
> Because the modified code is also used for the KVM_GET_MSRS device ioctl(),
> explicitly check @vcpu is non-null before attempting to load guest state.
> The XSAVE-managed MSRs cannot be retrieved via the device ioctl() without
> loading guest FPU state (which doesn't exist).
> 
> Note that guest_cpuid_has() is not queried as host userspace is allowed to
> access MSRs that have not been exposed to the guest, e.g. it might do
> KVM_SET_MSRS prior to KVM_SET_CPUID2.
> 
> The two helpers are put here in order to manifest accessing xsave-managed
> MSRs requires special check and handling to guarantee the correctness of
> read/write to the MSRs.
> 
> Co-developed-by: Yang Weijiang <weijiang.yang@...el.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> Tested-by: Mathias Krause <minipli@...ecurity.net>
> Tested-by: John Allen <john.allen@....com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Signed-off-by: Chao Gao <chao.gao@...el.com>
> [sean: drop S_CET, add big comment, move accessors to x86.c]
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>