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From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	David Laight <David.Laight@ACULAB.COM>,
	Arvind Sankar <nivedita@alum.mit.edu>
Subject: [PATCH v2 1/2] KVM: x86/mmu: Reorder the reserved bit check in prefetch_invalid_gpte()
Date: Thu,  9 Jan 2020 15:06:39 -0800
Message-ID: <20200109230640.29927-2-sean.j.christopherson@intel.com> (raw)
In-Reply-To: <20200109230640.29927-1-sean.j.christopherson@intel.com>

Move the !PRESENT and !ACCESSED checks in FNAME(prefetch_invalid_gpte)
above the call to is_rsvd_bits_set().  For a well behaved guest, the
!PRESENT and !ACCESSED are far more likely to evaluate true than the
reserved bit checks, and they do not require additional memory accesses.

 Dump of assembler code for function paging32_prefetch_invalid_gpte:
   0x0000000000044240 <+0>:     callq  0x44245 <paging32_prefetch_invalid_gpte+5>
   0x0000000000044245 <+5>:     mov    %rcx,%rax
   0x0000000000044248 <+8>:     shr    $0x7,%rax
   0x000000000004424c <+12>:    and    $0x1,%eax
   0x000000000004424f <+15>:    lea    0x0(,%rax,4),%r8
   0x0000000000044257 <+23>:    add    %r8,%rax
   0x000000000004425a <+26>:    mov    %rcx,%r8
   0x000000000004425d <+29>:    and    0x120(%rsi,%rax,8),%r8
   0x0000000000044265 <+37>:    mov    0x170(%rsi),%rax
   0x000000000004426c <+44>:    shr    %cl,%rax
   0x000000000004426f <+47>:    and    $0x1,%eax
   0x0000000000044272 <+50>:    or     %rax,%r8
   0x0000000000044275 <+53>:    jne    0x4427c <paging32_prefetch_invalid_gpte+60>
   0x0000000000044277 <+55>:    test   $0x1,%cl
   0x000000000004427a <+58>:    jne    0x4428a <paging32_prefetch_invalid_gpte+74>
   0x000000000004427c <+60>:    mov    %rdx,%rsi
   0x000000000004427f <+63>:    callq  0x44080 <drop_spte>
   0x0000000000044284 <+68>:    mov    $0x1,%eax
   0x0000000000044289 <+73>:    retq
   0x000000000004428a <+74>:    xor    %eax,%eax
   0x000000000004428c <+76>:    and    $0x20,%ecx
   0x000000000004428f <+79>:    jne    0x44289 <paging32_prefetch_invalid_gpte+73>
   0x0000000000044291 <+81>:    mov    %rdx,%rsi
   0x0000000000044294 <+84>:    callq  0x44080 <drop_spte>
   0x0000000000044299 <+89>:    mov    $0x1,%eax
   0x000000000004429e <+94>:    jmp    0x44289 <paging32_prefetch_invalid_gpte+73>
 End of assembler dump.

 Dump of assembler code for function paging32_prefetch_invalid_gpte:
   0x0000000000044240 <+0>:     callq  0x44245 <paging32_prefetch_invalid_gpte+5>
   0x0000000000044245 <+5>:     test   $0x1,%cl
   0x0000000000044248 <+8>:     je     0x4424f <paging32_prefetch_invalid_gpte+15>
   0x000000000004424a <+10>:    test   $0x20,%cl
   0x000000000004424d <+13>:    jne    0x4425d <paging32_prefetch_invalid_gpte+29>
   0x000000000004424f <+15>:    mov    %rdx,%rsi
   0x0000000000044252 <+18>:    callq  0x44080 <drop_spte>
   0x0000000000044257 <+23>:    mov    $0x1,%eax
   0x000000000004425c <+28>:    retq
   0x000000000004425d <+29>:    mov    %rcx,%rax
   0x0000000000044260 <+32>:    mov    (%rsi),%rsi
   0x0000000000044263 <+35>:    shr    $0x7,%rax
   0x0000000000044267 <+39>:    and    $0x1,%eax
   0x000000000004426a <+42>:    lea    0x0(,%rax,4),%r8
   0x0000000000044272 <+50>:    add    %r8,%rax
   0x0000000000044275 <+53>:    mov    %rcx,%r8
   0x0000000000044278 <+56>:    and    0x120(%rsi,%rax,8),%r8
   0x0000000000044280 <+64>:    mov    0x170(%rsi),%rax
   0x0000000000044287 <+71>:    shr    %cl,%rax
   0x000000000004428a <+74>:    and    $0x1,%eax
   0x000000000004428d <+77>:    mov    %rax,%rcx
   0x0000000000044290 <+80>:    xor    %eax,%eax
   0x0000000000044292 <+82>:    or     %rcx,%r8
   0x0000000000044295 <+85>:    je     0x4425c <paging32_prefetch_invalid_gpte+28>
   0x0000000000044297 <+87>:    mov    %rdx,%rsi
   0x000000000004429a <+90>:    callq  0x44080 <drop_spte>
   0x000000000004429f <+95>:    mov    $0x1,%eax
   0x00000000000442a4 <+100>:   jmp    0x4425c <paging32_prefetch_invalid_gpte+28>
 End of assembler dump.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
 arch/x86/kvm/mmu/paging_tmpl.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index b53bed3c901c..1fde6a1c506d 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -175,9 +175,6 @@ static bool FNAME(prefetch_invalid_gpte)(struct kvm_vcpu *vcpu,
 				  struct kvm_mmu_page *sp, u64 *spte,
 				  u64 gpte)
-	if (is_rsvd_bits_set(vcpu->arch.mmu, gpte, PT_PAGE_TABLE_LEVEL))
-		goto no_present;
 	if (!FNAME(is_present_gpte)(gpte))
 		goto no_present;
@@ -186,6 +183,9 @@ static bool FNAME(prefetch_invalid_gpte)(struct kvm_vcpu *vcpu,
 	    !(gpte & PT_GUEST_ACCESSED_MASK))
 		goto no_present;
+	if (is_rsvd_bits_set(vcpu->arch.mmu, gpte, PT_PAGE_TABLE_LEVEL))
+		goto no_present;
 	return false;

  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-09 23:06 [PATCH v2 0/2] KVM: x86/mmu: Optimize rsvd pte checks Sean Christopherson
2020-01-09 23:06 ` Sean Christopherson [this message]
2020-01-10 11:37   ` [PATCH v2 1/2] KVM: x86/mmu: Reorder the reserved bit check in prefetch_invalid_gpte() Vitaly Kuznetsov
2020-01-09 23:06 ` [PATCH v2 2/2] KVM: x86/mmu: Micro-optimize nEPT's bad memptype/XWR checks Sean Christopherson
2020-01-10 11:37   ` Vitaly Kuznetsov
2020-01-10 16:04     ` Sean Christopherson
2020-01-10 16:13       ` David Laight
2020-01-15 18:17       ` Paolo Bonzini

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