linux-kernel - RE: [PATCH v2 2/2] KVM: x86/mmu: Micro-optimize nEPT's bad memptype/XWR checks

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Date:   Fri, 10 Jan 2020 16:13:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sean Christopherson' <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>
Subject: RE: [PATCH v2 2/2] KVM: x86/mmu: Micro-optimize nEPT's bad
 memptype/XWR checks

From: Sean Christopherson <sean.j.christopherson@...el.com>
> Sent: 10 January 2020 16:05
...
> Similar to your suggestion, but it avoids evaluating __is_bad_mt_xwr() if
> reserved bits are set, which is admittedly rare.
> 
> 	return __is_rsvd_bits_set(&mmu->guest_rsvd_check, gpte, level)
> #if PTTYPE == PTTYPE_EPT
> 	       || __is_bad_mt_xwr(&mmu->guest_rsvd_check, gpte)
> #endif
> 	       ;

Or:
	return __is_rsvd_bits_set(&mmu->guest_rsvd_check, gpte, level) ||
		(PTTYPE == PTTYPE_EPT && __is_bad_mt_xwr(&mmu->guest_rsvd_check, gpte));

Relying in the compiler to optimise it away.

	David

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