linux-kernel - Re: [PATCH RFC v1 02/11] x86/fpu/xstate: Introduce xstate order table and accessor macro

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Message-ID: <Z8DY7P6UJFyCg47Z@gmail.com>
Date: Thu, 27 Feb 2025 22:28:12 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: bp@...en8.de, chang.seok.bae@...el.com, dave.hansen@...el.com,
	dave.hansen@...ux.intel.com, linux-kernel@...r.kernel.org,
	mingo@...hat.com, tglx@...utronix.de, x86@...nel.org
Subject: Re: [PATCH RFC v1 02/11] x86/fpu/xstate: Introduce xstate order
 table and accessor macro


* Andrew Cooper <andrew.cooper3@...rix.com> wrote:

> > There's no incompatibility for a default-disabled feature that gets 
> > enabled by an AVX-aware host kernel and by AVX-aware guest kernels. 
> > What ABI would be broken?
> 
> I don't understand your question.
>
> XSAVE, and details about in CPUID, are a stated ABI (given in the SDM 
> and APM), and available in userspace, including for userpace to write 
> into a file/socket and interpret later (this is literally how we 
> migrate VMs between different hosts).
> 
> You simply redefine what %xcr0.bnd_* (a.k.a. XFEATURES 3 and 4) mean, 
> irrespective of what a guest kernel thinks it can get away with.

XFEATURES bits 3 and 4 are zero by default in the CPU, so the previous 
ABI promise has been fully met: MPX is unavailable and disabled.

I propose a new addition, an extension of functionality: if a new CPUID 
bit indicates it, and a new MSR is written, XFEATURES bit 3 becomes 
active again - and the MPX area is now used by AVX. Obviously only 
AVX-aware host and guest kernels would enable AVX.

Thanks,

	Ingo