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

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Message-ID: <ea20d47e-88b9-46ab-9893-26bcf262d8b0@citrix.com>
Date: Thu, 27 Feb 2025 21:10:19 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Ingo Molnar <mingo@...nel.org>
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

On 27/02/2025 8:06 pm, Ingo Molnar wrote:
> * Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>> What options #1 and #4 will cause is the virt people to come after 
>> you with sharp implements for creating incompatibilities in an ABI.  
>> The XFEATUREs are the tag(ish) of the union that is the xsave buffer.
> 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.

~Andrew