linux-kernel - [PATCH 12/12] KVM: MMU: micro-optimize gpte

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Message-Id: <1456319873-34182-13-git-send-email-pbonzini@redhat.com>
Date:	Wed, 24 Feb 2016 14:17:53 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	yoshikawa_takuya_b1@....ntt.co.jp, guangrong.xiao@...ux.intel.com,
	mtosatti@...hat.com
Subject: [PATCH 12/12] KVM: MMU: micro-optimize gpte_access

Avoid AND-NOT, most x86 processor lack an instruction for it.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/paging_tmpl.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 2e019b103249..3c77bdd443d9 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -189,8 +189,11 @@ static inline unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, u64 gpte)
 		((gpte & VMX_EPT_EXECUTABLE_MASK) ? ACC_EXEC_MASK : 0) |
 		ACC_USER_MASK;
 #else
-	access = (gpte & (PT_WRITABLE_MASK | PT_USER_MASK)) | ACC_EXEC_MASK;
-	access &= ~(gpte >> PT64_NX_SHIFT);
+	BUILD_BUG_ON(ACC_EXEC_MASK != PT_PRESENT_MASK);
+	BUILD_BUG_ON(ACC_EXEC_MASK != 1);
+	access = gpte & (PT_WRITABLE_MASK | PT_USER_MASK | PT_PRESENT_MASK);
+	/* Combine NX with P (which is set here) to get ACC_EXEC_MASK.  */
+	access ^= (gpte >> PT64_NX_SHIFT);
 #endif
 
 	return access;
-- 
1.8.3.1