On Tue, May 21, 2024 at 12:48:16PM +0200, Jiri Olsa wrote:

hi,
as part of the effort on speeding up the uprobes [0] coming with
return uprobe optimization by using syscall instead of the trap
on the uretprobe trampoline.

I understand this provides an optimization on x86. I believe primary reason
is syscall is straight-line microcode and short sequence while trap delivery
still does all the GDT / IDT and segmentation checks and it makes delivery
of the trap slow.

So doing syscall improves that. Although it seems x86 is going to get rid of 
that as part of FRED [1, 2]. And linux kernel support for FRED is already upstream [2].
So I am imagining x86 hardware already exists with FRED support.

On other architectures, I believe trap delivery for breakpoint instruction
is same as syscall instruction.

Given that x86 trap delivery is pretty much going following the suit here and
intend to make trap delivery cost similar to syscall delivery.

Sorry for being buzzkill here but ...
Is it worth introducing this syscall which otherwise has no use on other arches
and x86 (and x86 kernel) has already taken steps to match trap delivery latency with
syscall latency would have similar cost?

Did you do any study of this on FRED enabled x86 CPUs?

[1] - https://www.intel.com/content/www/us/en/content-details/780121/flexible-return-and-event-delivery-fred-specification.html
[2] - https://docs.kernel.org/arch/x86/x86_64/fred.html

The speed up depends on instruction type that uprobe is installed
and depends on specific HW type, please check patch 1 for details.