Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

[H. Peter Anvin]

On 10/01/2014 09:04 AM, Andy Lutomirski wrote:
> 
> Agner Fog's tables for Sandy Bridge have 9 uops for popf and
> reciprocal throughput 18.  sti isn't listed for Sandy Bridge or
> anything similar, but cld is 3 uops with reciprocal throughput 4.
> Also, popf accesses rsp, and the sysenter code is very heavy on stack
> manipulation.
> 

It does a stack operation.  Newer CPUs optimize stack accesses pretty
heavily.  That doesn't mean back-to-back push/pop are all that
optimized, I wonder if it would help separating them.  popf is unlikely
to ever be all that fast.

	-hpa

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