linux-kernel - [PATCH v2 1/2] KVM: x86/mmu: Reorder the reserved bit check in prefetch_invalid

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Date:   Thu,  9 Jan 2020 15:06:39 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        David Laight <David.Laight@...LAB.COM>,
        Arvind Sankar <nivedita@...m.mit.edu>
Subject: [PATCH v2 1/2] KVM: x86/mmu: Reorder the reserved bit check in prefetch_invalid_gpte()

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.

Before:
 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.

After:
 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@...el.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;
 
 no_present:
-- 
2.24.1