linux-kernel - Re: [PATCH] x86/bhi: avoid hardware mitigation for 'spectre

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Message-ID: <20240912162440.be23sgv5v5ojtf3q@desk>
Date: Thu, 12 Sep 2024 09:24:40 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Jon Kohler <jon@...anix.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"kvm @ vger . kernel . org" <kvm@...r.kernel.org>,
	chao.gao@...el.com
Subject: Re: [PATCH] x86/bhi: avoid hardware mitigation for
 'spectre_bhi=vmexit'

On Thu, Sep 12, 2024 at 03:44:38PM +0000, Jon Kohler wrote:
> > It is only worth implementing the long sequence in VMEXIT_ONLY mode if it is
> > significantly better than toggling the MSR.
> 
> Thanks for the pointer! I hadn’t seen that second sequence. I’ll do measurements on
> three cases and come back with data from an SPR system.
> 1. as-is (wrmsr on entry and exit)
> 2. Short sequence (as a baseline)
> 3. Long sequence

I wonder if virtual SPEC_CTRL feature introduced in below series can
provide speedup, as it can replace the MSR toggling with faster VMCS
operations:

  https://lore.kernel.org/kvm/20240410143446.797262-1-chao.gao@intel.com/

Adding Chao for their opinion.