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

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Message-ID: <1a73d33b-e943-4ddd-9373-4678ff85c90a@citrix.com>
Date: Thu, 27 Feb 2025 19:34:34 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: dave.hansen@...el.com
Cc: bp@...en8.de, chang.seok.bae@...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 2/27/25 19:03, Dave Hansen wrote:

> On 2/27/25 10:44, Chang S. Bae wrote:
>> The kernel has largely assumed that higher xstate component numbers
>> correspond to later offsets in the buffer. However, this assumption
>> does not hold for the non-compacted format, where a newer state
>> component may have a lower offset.
> Maybe "no longer holds" instead of "does not hold".
>
> This never happened before APX, right?

I'm afraid that AMD beat you there by a decade with LWP, index 63 but
also overlaps the MPX state.

Except LWP support never became mainstream, and it also got sacrificed
to make room for IBPB in microcode, so you can safely ignore it[1].

~Andrew

[1] Other than for competitions of who did the silly thing first.