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: <9beb7ca4-7bcf-45f1-aefa-f8e6e8122ede@intel.com>
Date: Thu, 15 Jan 2026 08:07:16 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Jim Mattson <jmattson@...gle.com>, Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, 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 1/6/26 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.