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

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Message-ID: <c3fbf18a4ec015039388617ed899db98272cf181.camel@amd.com>
Date: Mon, 4 Nov 2024 08:57:29 +0000
From: "Shah, Amit" <Amit.Shah@....com>
To: "pawan.kumar.gupta@...ux.intel.com" <pawan.kumar.gupta@...ux.intel.com>
CC: "corbet@....net" <corbet@....net>, "kvm@...r.kernel.org"
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Subject: Re: [PATCH 1/2] x86: cpu/bugs: add support for AMD ERAPS feature

On Thu, 2024-10-31 at 16:03 -0700, Pawan Gupta wrote:
> On Thu, Oct 31, 2024 at 04:39:24PM +0100, Amit Shah wrote:
> > From: Amit Shah <amit.shah@....com>
> > 
> > Remove explicit RET stuffing / filling on VMEXITs and context
> > switches on AMD CPUs with the ERAPS feature (Turin+).
> > 
> > With the Enhanced Return Address Prediction Security feature,  any
> > hardware TLB flush results in flushing of the RSB (aka RAP in AMD
> > spec).
> > This guarantees an RSB flush across context switches.
> 
> Is it that the mov to CR3 triggers the RSB flush?

The INVPCID instruction, that causes the TLB flush, is the trigger
here.

> > Feature documented in AMD PPR 57238.
> 
> I couldn't find ERAPS feature description here, I could only manage
> to find
> the bit position:
> 
> 24 	ERAPS. Read-only. Reset: 1. Indicates support for enhanced
> return
> 	address predictor security.
> 
> Could you please point me to the document/section where this is
> described?

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/

Thanks,
		Amit