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

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Message-ID: <20250815105908.GB3245006@noisy.programming.kicks-ass.net>
Date: Fri, 15 Aug 2025 12:59:08 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, 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 Fri, Aug 15, 2025 at 11:43:11AM +0100, Andrew Cooper wrote:

> >> I've had to use cs prefixed jcc.d32, because our older toolchains don't
> >> like the ,pn notation.
> > And then I forgot to move that cs prefix around in the bhi1 case...
> > fixed that.
> 
> Dare I ask what ,pn notation is?  It's not only the older toolchains
> that don't know it :)

Some fancy notation for the CS/DS branch hints.

CS Jcc, decodes to Jcc,pn for non-taken
DS Jcc, decodes to Jcc,pt for taken