linux-kernel - Re: [PATCH v4 06/11] x86/vmscape: Move mitigation selection to a switch()

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Message-ID: <20251125174541.7lgjdfwws4ios4vg@desk>
Date: Tue, 25 Nov 2025 09:45:41 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Nikolay Borisov <nik.borisov@...e.com>
Cc: x86@...nel.org, David Kaplan <david.kaplan@....com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Asit Mallick <asit.k.mallick@...el.com>,
	Tao Zhang <tao1.zhang@...el.com>
Subject: Re: [PATCH v4 06/11] x86/vmscape: Move mitigation selection to a
 switch()

On Tue, Nov 25, 2025 at 12:19:32PM +0200, Nikolay Borisov wrote:
> > FEATURE_IBPB check is still needed for VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER.
> > I don't think we can drop that.
> 
> But if X86_FEATURE_IBPB is not present then all branches boil down to
> setting the mitigation to NONE. What I was suggesting is to not remove the
> that check at the top.

BHB_CLEAR mitigation is still possible without IBPB, with that IBPB check cannot
be at the top. This patch prepares for adding BHB_CLEAR support.

Sure I can delay moving the IBPB check to later patch, but the intent of
splitting the patches was to keep the patch that move the existing logic
separate from the one that adds a new mitigation.