Re: 8aeb879baf12 - significant system call latency regression, bisected

[H. Peter Anvin]

On 2026-06-16 00:53, Peter Zijlstra wrote:
> On Sun, Jun 14, 2026 at 07:07:50PM -0700, H. Peter Anvin wrote:
> 
>> It uses the same hack as the Makefile to deal with function alignment with a
>> prefix: it adds unnecessary NOPs so that the pre-alignment and
>> post-alignment are the same. At the end of the day this really ought to be
>> fixed in gcc.
> 
> And clang, but I don't think they can, it wrecks the 'ABI' they have in
> place with the current set of arguments. Which I agree is somewhat
> unfortunate, but it is what it is.
> 
> 
> This more or less works by accident, in general your align_func() macro
> is horrendously broken when you consider kCFI. By changing the
> patchable_function_entry attribute like this, the kCFI hash ends up at a
> different location and things go side-ways really really fast.
> 

OK, I guess I'm still wondering about this. Is there a difference between the
kCFI hash and the __pfx symbol?

> The only reason it works here is that this function is never indirectly
> called and so the kCFI ABI violation is immaterial.
Ironically, as you post in your later patch, it is arguably better to have it
always fail if it never should be called that way ;)

That being said, I have filed a gcc bug report:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125892

	-hpa