linux-kernel - Re: [PATCH v3 2/6] x86/bugs: Use SBPB in __write

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Message-ID: <5fnlcvhbmgtfxa7ivg6lks3vcnvcrrufl7xe2p2ifzhzkb65qo@2vgpex7e3g6b>
Date: Wed, 2 Apr 2025 19:12:32 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, amit@...nel.org, 
	kvm@...r.kernel.org, amit.shah@....com, bp@...en8.de, tglx@...utronix.de, 
	peterz@...radead.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
Subject: Re: [PATCH v3 2/6] x86/bugs: Use SBPB in __write_ibpb() if applicable

On Wed, Apr 02, 2025 at 03:41:25PM -0500, Tom Lendacky wrote:
> On 4/2/25 13:19, Josh Poimboeuf wrote:
> > __write_ibpb() does IBPB, which (among other things) flushes branch type
> > predictions on AMD.  If the CPU has SRSO_NO, or if the SRSO mitigation
> > has been disabled, branch type flushing isn't needed, in which case the
> > lighter-weight SBPB can be used.
> 
> Maybe add something here that indicates the x86_pred_cmd variable tracks
> this optimization so switch to using that variable vs the hardcoded IBPB?

Indeed, adding a second paragraph to clarify that:

  x86/bugs: Use SBPB in write_ibpb() if applicable

  write_ibpb() does IBPB, which (among other things) flushes branch type
  predictions on AMD.  If the CPU has SRSO_NO, or if the SRSO mitigation
  has been disabled, branch type flushing isn't needed, in which case the
  lighter-weight SBPB can be used.

  The 'x86_pred_cmd' variable already keeps track of whether IBPB or SBPB
  should be used.  Use that instead of hardcoding IBPB.

-- 
Josh