linux-kernel - Re: [PATCH v5 3/3] x86/bugs: Add 'spectre

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Date: Wed, 8 May 2024 22:24:47 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Nikolay Borisov <nik.borisov@...e.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alexandre Chartre <alexandre.chartre@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	KP Singh <kpsingh@...nel.org>, Waiman Long <longman@...hat.com>,
	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
	Maksim Davydov <davydov-max@...dex-team.ru>
Subject: Re: [PATCH v5 3/3] x86/bugs: Add 'spectre_bhi=vmexit' cmdline option

On Wed, May 08, 2024 at 06:10:21PM +0300, Nikolay Borisov wrote:
> > @@ -1659,19 +1662,22 @@ static void __init bhi_select_mitigation(void)
> >   			return;
> >   	}
> > +	/* Mitigate in hardware if supported */
> >   	if (spec_ctrl_bhi_dis())
> >   		return;
> >   	if (!IS_ENABLED(CONFIG_X86_64))
> >   		return;
> > -	/* Mitigate KVM by default */
> > -	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
> > -	pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit\n");
> > +	if (bhi_mitigation == BHI_MITIGATION_VMEXIT_ONLY) {
> > +		pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit only\n");
> > +		setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
> > +		return;
> > +	}
> 
> nit: How about setting CLEAR_BHB_LOOP_ON_VMEXIT unconditionally, then
> afterwards checking if MITIGATION_VMEXIT_ONLY is set and if yes simply
> return, that way you don't duplicate the setup of the VMEXIT code

I think the duplication actually makes it more readable.  In both cases
it puts the setting of the features together along with the
corresponding pr_info().

-- 
Josh