linux-kernel - Re: [PATCH v2 0/4] x86, fpu/kvm: fix crash with AMX

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Message-ID: <CABgObfYk-PxxGOj3az26=tt-p7_qu=eFhgdjKFqva7Stui9HYA@mail.gmail.com>
Date: Thu, 15 Jan 2026 14:49:34 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Borislav Petkov <bp@...en8.de>
Cc: "Kernel Mailing List, Linux" <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>, 
	Sean Christopherson <seanjc@...gle.com>, "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH v2 0/4] x86, fpu/kvm: fix crash with AMX

Il gio 15 gen 2026, 13:22 Borislav Petkov <bp@...en8.de> ha scritto:
>
> On Thu, Jan 01, 2026 at 10:05:12AM +0100, Paolo Bonzini wrote:
> > Fix a possible host panic, due to an unexpected #NM, when a KVM guest
> > is using AMX features.
> >
> > The guest's XFD value, which is stored in fpstate->xfd, is used for both
> > guest execution and host XSAVE operations.
>
> This already sounds weird. Why?

Because the state of disabled components is undefined anyway. There's
no point in making all host XSAVEs more expensive, even when the TMM
registers aren't in use by the guest (which is going to be most of the
time, likely).

> Why don't we carry separate XFD copies - guest and host - which we use for the
> guest and the host, respectively?

That was exactly what I did in v1, but it's more code and less efficient too.

Paolo

>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette
>