linux-kernel - Re: [PATCH v3 1/1] x86/fred: Remove ENDBR64 from FRED entry points

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Message-ID: <d0cac7d8-3036-4241-b11c-f005224daefd@zytor.com>
Date: Wed, 13 Aug 2025 14:57:55 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Dave Hansen <dave.hansen@...el.com>, "Xin Li (Intel)" <xin@...or.com>,
        linux-kernel@...r.kernel.org
Cc: luto@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, jmill@....edu,
        peterz@...radead.org, andrew.cooper3@...rix.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v3 1/1] x86/fred: Remove ENDBR64 from FRED entry points

On 2025-08-13 14:07, Dave Hansen wrote:
> On 7/15/25 23:33, Xin Li (Intel) wrote:
>> The FRED specification has been changed in v9.0 to state that there
>> is no need for FRED event handlers to begin with ENDBR64, because
>> in the presence of supervisor indirect branch tracking, FRED event
>> delivery does not enter the WAIT_FOR_ENDBRANCH state.
>>
>> As a result, remove ENDBR64 from FRED entry points.
> 
> So, the spec is being updated or has been updated to reflect the new
> architecture, right?
> 
> Remind me, are there no FRED systems out in the wild today, so we don't
> have to worry about breaking anything?

Correct. The FRED v9 spec contains this change.

All production hardware and late pre-production hardware will have this.

	-hpa