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

H. Peter Anvin
2014-10-01T16:17:56Z

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