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

[Peter Zijlstra]

On Thu, Jun 18, 2026 at 04:05:37PM -0700, H. Peter Anvin wrote:
> 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

For GCC you also need these unmerged patches: 

  https://lore.kernel.org/r/20260618204530.work.910-kees@xxxxxxxxxx

And you seem to have forgotten about llvm/clang.