linux-kernel - Re: [PATCH v16 09/51] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs

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Message-ID: <aNF8JMN71Bibp24U@google.com>
Date: Mon, 22 Sep 2025 09:41:08 -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>, Xiaoyao Li <xiaoyao.li@...el.com>, 
	Maxim Levitsky <mlevitsk@...hat.com>, Zhang Yi Z <yi.z.zhang@...ux.intel.com>, Xin Li <xin@...or.com>
Subject: Re: [PATCH v16 09/51] KVM: x86: Load guest FPU state when access
 XSAVE-managed MSRs

On Mon, Sep 22, 2025, Binbin Wu wrote:
> 
> 
> On 9/20/2025 6:32 AM, Sean Christopherson wrote:
> [...]
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 3e66d8c5000a..ae402463f991 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -136,6 +136,9 @@ static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
> >   static void __get_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
> >   static DEFINE_MUTEX(vendor_module_lock);
> > +static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu);
> > +static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu);
> > +
> >   struct kvm_x86_ops kvm_x86_ops __read_mostly;
> >   #define KVM_X86_OP(func)					     \
> > @@ -3801,6 +3804,67 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
> >   	mark_page_dirty_in_slot(vcpu->kvm, ghc->memslot, gpa_to_gfn(ghc->gpa));
> >   }
> > +/*
> > + * Returns true if the MSR in question is managed via XSTATE, i.e. is context
> > + * switched with the rest of guest FPU state.  Note!  S_CET is _not_ context
> > + * switched via XSTATE even though it _is_ saved/restored via XSAVES/XRSTORS.
> > + * Because S_CET is loaded on VM-Enter and VM-Exit via dedicated VMCS fields,
> > + * the value saved/restored via XSTATE is always the host's value.  That detail
> > + * is _extremely_ important, as the guest's S_CET must _never_ be resident in
> > + * hardware while executing in the host.  Loading guest values for U_CET and
> > + * PL[0-3]_SSP while executing in the kernel is safe, as U_CET is specific to
> > + * userspace, and PL[0-3]_SSP are only consumed when transitioning to lower
> > + * privilegel levels, i.e. are effectively only consumed by userspace as well.
> 
> s/privilegel/privilege[...]

Fixed up, thanks!