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

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Message-ID: <aMqt46hxeKxCxkmq@google.com>
Date: Wed, 17 Sep 2025 05:47:45 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, 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>, 
	Xiaoyao Li <xiaoyao.li@...el.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 Tue, Sep 16, 2025, Binbin Wu wrote:
> > +/*
> > + * Lock and/or reload guest FPU and access xstate MSRs. For accesses initiated
> 
> 
> Lock is unconditional and reload is conditional.
> "and/or" seems not accurate?

Agreed.  This?

/*
 * Lock andr (re)load guest FPU and access xstate MSRs. For accesses initiated
 * by host, guest FPU is loaded in __msr_io(). For accesses initiated by guest,
 * guest FPU should have been loaded already.
 */

> 
> > + * by host, guest FPU is loaded in __msr_io(). For accesses initiated by guest,
> > + * guest FPU should have been loaded already.
> > + */
> > +static __always_inline void kvm_access_xstate_msr(struct kvm_vcpu *vcpu,
> > +						  struct msr_data *msr_info,
> > +						  int access)
> > +{
> > +	BUILD_BUG_ON(access != MSR_TYPE_R && access != MSR_TYPE_W);
> > +
> > +	KVM_BUG_ON(!is_xstate_managed_msr(vcpu, msr_info->index), vcpu->kvm);
> > +	KVM_BUG_ON(!vcpu->arch.guest_fpu.fpstate->in_use, vcpu->kvm);
> > +
> > +	kvm_fpu_get();
> > +	if (access == MSR_TYPE_R)
> > +		rdmsrq(msr_info->index, msr_info->data);
> > +	else
> > +		wrmsrq(msr_info->index, msr_info->data);
> > +	kvm_fpu_put();
> > +}
> > +
> > +static __maybe_unused void kvm_set_xstate_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > +{
> > +	kvm_access_xstate_msr(vcpu, msr_info, MSR_TYPE_W);
> > +}
> > +
> > +static __maybe_unused void kvm_get_xstate_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > +{
> > +	kvm_access_xstate_msr(vcpu, msr_info, MSR_TYPE_R);
> > +}
> > +
> >   int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >   {
> >   	u32 msr = msr_info->index;
> > @@ -4551,11 +4614,25 @@ static int __msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs *msrs,
> >   		    int (*do_msr)(struct kvm_vcpu *vcpu,
> >   				  unsigned index, u64 *data))
> >   {
> > +	bool fpu_loaded = false;
> >   	int i;
> > -	for (i = 0; i < msrs->nmsrs; ++i)
> > +	for (i = 0; i < msrs->nmsrs; ++i) {
> > +		/*
> > +		 * If userspace is accessing one or more XSTATE-managed MSRs,
> > +		 * temporarily load the guest's FPU state so that the guest's
> > +		 * MSR value(s) is resident in hardware, i.e. so that KVM can
> 
> Using "i.e." and "so that" together feels repetitive.[...]

		/*
		 * If userspace is accessing one or more XSTATE-managed MSRs,
		 * temporarily load the guest's FPU state so that the guest's
		 * MSR value(s) is resident in hardware and thus can be accessed
		 * via RDMSR/WRMSR.
		 */