linux-kernel - Re: [PATCH] x86/fred: Fix 64bit identifier in fred

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Message-ID: <02510395-25c1-4aa6-a836-85151927d2f7@zytor.com>
Date: Thu, 14 Aug 2025 20:55:48 -0700
From: Xin Li <xin@...or.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc: "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Subject: Re: [PATCH] x86/fred: Fix 64bit identifier in fred_ss

On 8/14/2025 12:04 PM, Andrew Cooper wrote:
> FRED can only be enabled in Long Mode.  This is the 64bit mode (as opposed to
> compatibility mode) identifier, rather than being something hard-wired at 1.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@...rix.com>
> ---
> CC: Xin Li <xin@...or.com>
> CC: "H. Peter Anvin" <hpa@...or.com>
> CC: Andy Lutomirski <luto@...nel.org>
> CC: Thomas Gleixner <tglx@...utronix.de>
> CC: Ingo Molnar <mingo@...hat.com>
> CC: Borislav Petkov <bp@...en8.de>
> CC: Dave Hansen <dave.hansen@...ux.intel.com>
> CC: x86@...nel.org
> CC: linux-kernel@...r.kernel.org
> 
> IIRC, this was wrong in an older version of the FRED spec.  I made the same
> mistake in Xen, and I didn't copy Linux's structure.
> 
> I don't thinks this warrants backporting, but the fixes tag is:
> 
> Fixes: 3c77bf02d0c0 ("x86/ptrace: Add FRED additional information to the pt_regs structure")
> ---


Oh right — to be precise, long mode (also known as IA-32e mode in the
Intel SDM) includes two sub-modes:

     1) Compatibility mode
     2) 64-bit mode

So, "long mode" is not the correct term in this context and should be
corrected.

Reviewed-by: Xin Li (Intel) <xin@...or.com>