linux-kernel - Re: [RFC PATCH v3 1/2] x86: cpu/bugs: add AMD ERAPS support; hardware flushes RSB

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Message-ID: <4601ca077c95393837eb40909c941a4d67bb04dd.camel@kernel.org>
Date: Mon, 02 Dec 2024 19:09:34 +0100
From: Amit Shah <amit@...nel.org>
To: Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, x86@...nel.org, linux-doc@...r.kernel.org
Cc: thomas.lendacky@....com, bp@...en8.de, tglx@...utronix.de, 
	peterz@...radead.org, jpoimboe@...nel.org,
 pawan.kumar.gupta@...ux.intel.com, 	corbet@....net, mingo@...hat.com,
 dave.hansen@...ux.intel.com, hpa@...or.com, 	seanjc@...gle.com,
 pbonzini@...hat.com, daniel.sneddon@...ux.intel.com, 	kai.huang@...el.com,
 sandipan.das@....com, boris.ostrovsky@...cle.com, 	Babu.Moger@....com,
 david.kaplan@....com, dwmw@...zon.co.uk, 	andrew.cooper3@...rix.com, Amit
 Shah <Amit.Shah@....com>
Subject: Re: [RFC PATCH v3 1/2] x86: cpu/bugs: add AMD ERAPS support;
 hardware flushes RSB

On Mon, 2024-12-02 at 09:26 -0800, Dave Hansen wrote:
> On 11/28/24 05:28, Amit Shah wrote:
> > From: Amit Shah <amit.shah@....com>
> > 
> > When Automatic IBRS is disabled, Linux flushed the RSB on every
> > context
> > switch.  This RSB flush is not necessary in software with the ERAPS
> > feature on Zen5+ CPUs that flushes the RSB in hardware on a context
> > switch (triggered by mov-to-CR3).
> > 
> > Additionally, the ERAPS feature also tags host and guest addresses
> > in
> > the RSB - eliminating the need for software flushing of the RSB on
> > VMEXIT.
> > 
> > Disable all RSB flushing by Linux when the CPU has ERAPS.
> > 
> > Feature mentioned in AMD PPR 57238.  Will be resubmitted once APM
> > is
> > public - which I'm told is imminent.
> 
> There was a _lot_ of discussion about this. But all of that
> discussion
> seems to have been trimmed out and it seems like we're basically back
> to: "this is new hardware supposed to mitigate SpectreRSB, thus it
> mitigates SpectreRSB."

Absolutely, I don't want that to get lost -- but I think that got
captured in Josh's rework patchset.  With that rework, I don't even
need this patchset for the hardware feature to work, because we now
rely on AutoIBRS to do the RSB clearing; and the hardware takes care of
AutoIBRS and ERAPS interaction in Zen5.

The only thing this patch now does is to handle the AutoIBRS-disabled
case -- which happens when SEV-SNP is turned on (i.e. let the hw clear
the RSB instead of stuffing it in Linux).

I can still include the summary of the discussion in this patch - I
just feel it isn't necessary with the rework.

		Amit