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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 12, 2026 at 06:45:06PM -0700, "H. Peter Anvin" (Intel) wrote:
> So I was trying to figure out a significant -- about 13% -- increase
> in system call latency between v7.0 and the current master, and it
> bisects down to:
> 
> 	8aeb879baf12 x86/kvm/vmx: Fix x86_64 CFI build
> 
> This is on Panther Lake (Core Ultra X7 358H) with FRED enabled. This
> is a bare metal boot, no KVM.
> 
> I'm personally extremely puzzled how this could possibly be related,
> and I will be investigating the possibility that this is a false
> bisect, but it is not a Heisenbug in any way; it has been extremely
> reproducible, and the difference is statistically valid by close to 10
> sigma. Futhermore, the bisection at least gave the appearance of
> stability.
> 
> Given how late in the cycle this is I wanted to send an alert sooner
> rather than later; I will update as I get more data.

Uhm, massive WTF indeed. I don't immediately see how this could possibly
affect a FRED host either, except perhaps in code layout.

I don't actually have a FRED capable machine, but have you tried running
one of those top-down perf things on it, to see where its hurting?