linux-kernel - Re: [RFC][PATCH] x86,ibt: Use UDB instead of 0xEA

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Message-ID: <7594ecd7-994d-4cb8-9f8a-b6c1d4b01e17@citrix.com>
Date: Fri, 15 Aug 2025 18:21:48 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: "H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, kees@...nel.org, alyssa.milburn@...el.com,
 scott.d.constable@...el.com, joao@...rdrivepizza.com,
 samitolvanen@...gle.com, nathan@...nel.org, alexei.starovoitov@...il.com,
 mhiramat@...nel.org, ojeda@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] x86,ibt: Use UDB instead of 0xEA

On 15/08/2025 6:14 pm, H. Peter Anvin wrote:
> On 2025-08-15 04:19, Andrew Cooper wrote:
>>> CS Jcc, decodes to Jcc,pn for non-taken
>>> DS Jcc, decodes to Jcc,pt for taken
>> Ah, thanks.  I was looking at the hex in one of the comments and still
>> couldn't figure it out.
>>
>> So with this notation, we also have the dual meaning of ,pt between the
>> P4 and LNC.  At least the encoding is the same.
>>
> What "dual meaning?"

Well, it has different semantics now it's been reintroduced in LNC. 
(Only has any effect on a prediction miss, and causes a proactive decode
resteer).

Sure, it's "just perf" so can be argued as "compatible behaviour", but
people caring about the ,pt / ,pn properties do need to know the different.

~Andrew