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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 14/02/18 13:04, zhenzhong.duan wrote:

2018年2月14日 20:21于 Andrew Cooper <andrew.cooper3@xxxxxxxxxx>写道：
>
> 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.)
>
Thanks for explain. But just curious does processor has ideas the whole register is zeroed and avoid a merge?

Yes.  See the Zeroing Idioms section in the optimisation manual.  Tracking which registers are zero helps breaks read-after-write data dependences in the pipeline.

~Andrew

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