Re: [PATCH 3/4] x86: open-code register save/restore in trace_hardirqs thunks

[Andy Lutomirski]

On Sat, Jan 10, 2015 at 12:42 PM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Sat, Jan 10, 2015 at 12:17:13PM -0800, Andy Lutomirski wrote:
>> I asked this once, and someone told me that push/pop has lower
>> throughput.  I find this surprising.
>
> Implicit dependency on %rsp probably. The MOVs allow you to start more
> stuff out-of-order I'd guess...

AIUI modern CPUs have fancy stack engines that match call/ret pairs,
and presumably they can speculate rsp values across multiple pushes
and pops very quickly.

Also, don't compilers generally use push and pop to save and restore
callee-saved registers?  I think that function calls are common enough
that the CPU vendors would have made these sequences fast.

--Andy

>
>> Tt could be worth adding a macro along the lines of pushq_cfi_save
>> that does the pushq_cfi and the CFI_REL_OFFSET.
>
> Yep, for balance.
>
> --
> Regards/Gruss,
>     Boris.
>
> Sent from a fat crate under my desk. Formatting is fine.
> --



-- 
Andy Lutomirski
AMA Capital Management, LLC
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