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: <43474e30-6a14-4ab1-aa2c-5f079503637d@redhat.com>
Date: Thu, 15 Jan 2026 17:12:22 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Dave Hansen <dave.hansen@...el.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 17:07, Dave Hansen wrote:
> On 1/6/26 09:56, Jim Mattson wrote:
>> Apologies. You're right. Though Intel is a bit coy, the only way to
>> interpret that section of the SDM is to conclude that the AMX state in
>> the CPU becomes undefined when XFD[18] is set.
> 
> I'll touch base with the folks that wrote that blurb. I'm a little
> nervous to interpret that "software should not..." blurb as a full
> architectural DANGER sign partly because it's in a "RECOMMENDATIONS FOR
> SYSTEM SOFTWARE" section.
> 
> 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?

Paolo