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

[Peter Zijlstra]

On Sun, Jun 14, 2026 at 07:07:50PM -0700, H. Peter Anvin wrote:

> PeterZ: at some point you and I talked about the following:
> 
> - Should x64_sys_call() be noinstr?

I still think it should be, yes. But I also think it wants __noendbr,
there is no sane reason you should ever be allowed to do an indirect
call to this.

Realistically, objtool will seal this function (scribble the ENDBR), but
really, it just shouldn't be there to begin with.

> - If so, any reason we can't inline it into do_syscall_64()?

Code gen, GCC makes a mess out of things if you do that. x64_sys_call()
now ends up being a giant pile of tail-calls. If you inline it into
do_syscall_x64() that goes out the window.

> - Since we no longer use the sys_call_table[] as a jump table,
>   do we actually need array_index_nospec()? in do_syscall_x64|32?

It would mean unconditionally disabling jump-tables -- at least for this
TU, but possibly for the whole thing (mixed compiler flags and LTO is a
pain you don't need IIRC).