linux-kernel - Re: [PATCH 1/2] x86: Remove dynamic NOP selection

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Date: Sat, 20 Jan 2024 06:58:56 +0000 (UTC)
From: Thorsten Glaser <tg@...ian.org>
To: Peter Zijlstra <peterz@...radead.org>
cc: x86@...nel.org, rostedt@...dmis.org, hpa@...or.com,
        torvalds@...uxfoundation.org, linux-kernel@...r.kernel.org,
        linux-toolchains@...r.kernel.org, jpoimboe@...hat.com,
        alexei.starovoitov@...il.com, mhiramat@...nel.org
Subject: Re: [PATCH 1/2] x86: Remove dynamic NOP selection

Peter Zijlstra dixit:

>-/* generic versions from gas
[…]
>-   3: leal 0x00(%esi),%esi
>-   4: leal 0x00(,%esi,1),%esi
>-   6: leal 0x00000000(%esi),%esi
>-   7: leal 0x00000000(,%esi,1),%esi

vs.

>+ * Generic 32bit nops from GAS:
[…]
>+ * 3: leal 0x0(%esi),%esi
>+ * 4: leal 0x0(%esi,%eiz,1),%esi
>+ * 5: leal %ds:0x0(%esi,%eiz,1),%esi
>+ * 6: leal 0x0(%esi),%esi
>+ * 7: leal 0x0(%esi,%eiz,1),%esi
>+ * 8: leal %ds:0x0(%esi,%eiz,1),%esi

I think there’s some mistake introduced. The BYTES_* are
identical for e.g. #7, but %eiz must be wrong, it’s not
a register. Indeed, gas (on Debian bullseye) does not
assemble that either.

(Awful AT&T syntax aside…)

bye,
//mirabilos
-- 
„Cool, /usr/share/doc/mksh/examples/uhr.gz ist ja ein Grund,
mksh auf jedem System zu installieren.“
	-- XTaran auf der OpenRheinRuhr, ganz begeistert
(EN: “[…]uhr.gz is a reason to install mksh on every system.”)