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Re: [Xen-devel] 回复: [PATCH] x86/spec_ctrl: Fix several bugs in SPEC_CTRL_ENTRY_FROM_INTR_IST

On 14/02/18 12:08, zhenzhong.duan wrote:
> > @@ -286,13 +286,13 @@
> >      setz %dl
> >      and %dl, STACK_CPUINFO_FIELD(use_shadow_spec_ctrl)(%r14)
> Is it safe to remove the 'xor %edx, %edx' above? setz set whole byte 1
> or 0.

It is safe, but it is not a good idea.

Using setz is an 8bit operation, which will suffer a register merge
stall in the pipeline as we know for certain at this point that the
upper bits of %edx are nonzero at this point.  (An encoding which
allowed setz %eax would have been far more useful in 64bit code.)


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