On Thu, Jul 04, 2013 at 10:58:29AM +0200, Denys Vlasenko wrote:
> The check sits in switch() statement which itself can check
> for opcode 0x90 far more efficiently.
>
> On assembler level, this change simply eliminates the following
> bit of code:
>
> 4c 8b a3 d8 00 00 00 mov 0xd8(%rbx),%r12
> 31 f6 xor %esi,%esi
> 48 89 df mov %rbx,%rdi
> e8 32 8c ff ff callq 490 <reg_read>
> 48 8d 83 78 01 00 00 lea 0x178(%rbx),%rax
> 83 8b 70 01 00 00 01 orl $0x1,0x170(%rbx)
> 83 8b 74 01 00 00 01 orl $0x1,0x174(%rbx)
> 49 39 c4 cmp %rax,%r12
> 0f 84 88 fa ff ff je 7304 <x86_emulate_insn+0x204>
>
> Signed-off-by: Denys Vlasenko <[email protected]>
> CC: Paolo Bonzini <[email protected]>
> CC: Avi Kivity <[email protected]>
> ---
> arch/x86/kvm/emulate.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 2bc1e81..7e6e74a 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4544,9 +4544,9 @@ special_insn:
> case 0x8d: /* lea r16/r32, m */
> ctxt->dst.val = ctxt->src.addr.mem.ea;
> break;
> - case 0x90 ... 0x97: /* nop / xchg reg, rax */
> - if (ctxt->dst.addr.reg == reg_rmw(ctxt, VCPU_REGS_RAX))
> - break;
> + case 0x90: /* nop */
> + break;
This does not work on 64bit and REX prefix.
> + case 0x91 ... 0x97: /* xchg reg, rax */
> rc = em_xchg(ctxt);
> break;
> case 0x98: /* cbw/cwde/cdqe */
> --
> 1.8.1.4
--
Gleb.
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