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SubjectRe: [PATCH v2 1/2] KVM: x86/mmu: Reorder the reserved bit check in prefetch_invalid_gpte()
Date
Sean Christopherson <sean.j.christopherson@intel.com> writes:

> 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@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;
>
> no_present:

FWIW,
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

--
Vitaly

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