Re: [PATCH v3 1/5] asm-generic: barrier: Add smp_cond_load_relaxed_timewait()

[Catalin Marinas]

On Mon, Aug 18, 2025 at 01:51:28PM +0200, Arnd Bergmann wrote:
> On Thu, Aug 14, 2025, at 15:00, Catalin Marinas wrote:
> > On Wed, Aug 13, 2025 at 06:29:37PM +0200, Arnd Bergmann wrote:
> >> On Wed, Aug 13, 2025, at 18:09, Catalin Marinas wrote:
> >> and virtual machines with CPU overcommit.
> >
> > Not sure it helps here. With vCPU overcommit, KVM enables WFE trapping
> > and the event stream no longer has any effect (it's not like it
> > interrupts the host).
> 
> I would expect a similar overhead for the WFE trapping as for the
> bare-metal hardware case: When the WFE traps, the host has to
> reschedule all guests that are in WFE periodically, while WFET
> with event stream disabled means this can be driven by an accurate
> host timer.

For WFIT, yes, this works as the hypervisor either gets an interrupt or
schedules a timer. It can tell what WFI* is going to be woken by.

With WFET, the hypervisor cannot tell if an event was generated by
another vCPU (e.g. clearing of the exclusive monitor) unless it does a
WFE itself. So it can't put the vCPU to sleep based on the WFET timeout.
IIUC, from kvm_handle_wfx(), the only difference is whether it returns
immediately to the vCPU if it timed out or tells the core KVM to
reschedule other vCPUs.

> > That said, my worry is that either broken hardware or software rely on
> > the event stream unknowingly, e.g. someone using WFE in a busy loop. And
> > for hardware errata, we've had a few where the wakeup events don't
> > propagate between clusters, though these we can toggle on a case by case
> > basis.
> 
> Don't we already support hardware without a functional architected
> timer even with? Those don't use the event stream today even when
> CONFIG_ARM_ARCH_TIMER_EVTSTREAM is enabled.

Maybe we still have such hardware around (e.g. errata) but it shouldn't
be the norm, especially if vendors try to follow the *BSA specs.

-- 
Catalin