linux-kernel - Re: [PATCH 1/4] x86/fpu: Clear XSTATE_BV[i] in save state whenever XFD[i]=1

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Message-ID: <c0651275-b233-42c3-b6ee-aee5ddcc0a05@intel.com>
Date: Thu, 15 Jan 2026 15:43:06 -0800
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: Dave Hansen <dave.hansen@...el.com>, Paolo Bonzini <pbonzini@...hat.com>,
	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>
CC: <seanjc@...gle.com>, <x86@...nel.org>, Thomas Gleixner
	<tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 1/4] x86/fpu: Clear XSTATE_BV[i] in save state whenever
 XFD[i]=1

On 1/15/2026 10:19 AM, Dave Hansen wrote:
> 
> What would folks think about making the SDM language stronger, or at
> least explicitly adding the language that setting XFD[i]=1 can lead to
> XINUSE[i] going from 1=>0. Kinda like the language that's already in
> "XRSTOR and the Init and Modified Optimizations", but specific to XFD:
> 
> 	If XFD[i] = 1 and XINUSE[i] = 1, state component i may be
> 	tracked as init; XINUSE[i] may be set to 0.
> 
> That would make it consistent with the KVM behavior. It might also give
> the CPU folks some additional wiggle room for new behavior.

Yeah, I saw that you quoted this sentence in the XFD section in your 
other response:

	If XSAVE, XSAVEC, XSAVEOPT, or XSAVES is saving the state
	component i, the instruction does not generate #NM when XCR0[i]
	= IA32_XFD[i] = 1; instead, it operates as if XINUSE[i] = 0 (and
	the state component was in its initial state)

Indeed, I do applaud the idea to clarify this behavior more explicitly 
right there.

Thanks,
Chang