Re: [PATCH v2 1/2] KVM: x86/mmu: Reorder the reserved bit check in prefetch_invalid_gpte()

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> 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@xxxxxxxxx>
> ---
>  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@xxxxxxxxxx>

-- 
Vitaly