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

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Message-ID: <aMsuiTIUlulepJly@google.com>
Date: Wed, 17 Sep 2025 14:56:25 -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 Wed, Sep 17, 2025, Sean Christopherson wrote:
> 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.
>  */

That's not very good either.

/*
 * Lock (and if necessary, re-load) the guest FPU, i.e. XSTATE, and access an
 * MSR that is managed via XSTATE.  Note, the caller is responsible for doing
 * the initial FPU load, this helper only ensures that guest state is resident
 * in hardware (the kernel can load its FPU state in IRQ context).
 */