linux-kernel - Re: [PATCH 1/2] x86: cpu/bugs: add support for AMD ERAPS feature

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Message-ID: <e9711ae1-b983-4f3e-89b4-513db62e4eef@citrix.com>
Date: Mon, 4 Nov 2024 14:52:48 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: amit.shah@....com
Cc: Babu.Moger@....com, David.Kaplan@....com, Sandipan.Das@....com,
 Thomas.Lendacky@....com, boris.ostrovsky@...cle.com, bp@...en8.de,
 corbet@....net, daniel.sneddon@...ux.intel.com, dave.hansen@...ux.intel.com,
 hpa@...or.com, jpoimboe@...nel.org, kai.huang@...el.com,
 kvm@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, mingo@...hat.com,
 pawan.kumar.gupta@...ux.intel.com, pbonzini@...hat.com,
 peterz@...radead.org, seanjc@...gle.com, tglx@...utronix.de, x86@...nel.org
Subject: Re: [PATCH 1/2] x86: cpu/bugs: add support for AMD ERAPS feature

> Unfortunately, that's all we have right now in the official
> documentation.
>
> I've put up some notes in
> https://amitshah.net/2024/11/eraps-reduces-software-tax-for-hardware-bugs/

I appreciate the attempt to get a few details out, but this is very
confused on bunch of details.

Most importantly, you've described Intel RSB underflows, but named it
AMD BTC.

"Retbleed" is two totally different things.   I begged the discoverers
to give it two names, and I also begged the x86 maintainers to not alias
them in Linux's view of the world, but alas.

AMD's BTC comes from a bad branch type prediction, and a late resteer
from the ret uop executing.   It has nothing to do with RAS/RSB
underflow conditions.

~Andrew