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: <dde77ed7-3fde-4b81-85be-68db82138454@intel.com>
Date: Thu, 15 Jan 2026 08:27:44 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Jim Mattson <jmattson@...gle.com>,
 Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH 1/4] x86/fpu: Clear XSTATE_BV[i] in save state whenever
 XFD[i]=1

On 1/15/26 08:12, Paolo Bonzini wrote:
...
>> I'm _sure_ they discussed tying XFD[i] and XINUSE[i] together and there
>> was a good reason they did not.
> 
> Is there anything that prevents an SMM handler (or more likely, an SMI
> transfer monitor) to do an XSAVE/XRSTOR and destroy tile data?

I think you're saying: let's assume XFD[18]=1 and XINUSE[18]=1 and
there's an SMI. The SMI handler does:

	XSAVE(RFBM=-1, &buf)
	... run some gunk
	XRSTOR(RFBM=-1, &buf)

to try and save everything. But, that XSAVE is subject to this behavior
from the SDM:

	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)

So 'buf' will end up having XFEATURES[18]=0. The XRSTOR will see
XFEATURES[18]=0 and set feature 18 to its init state, effectively
zapping its contents.

I guess the only thing preventing that in practice is the lack of XSAVE
use in SMM handlers. But I see your point.