ARM WFET application scenario consultation

[Catalin Marinas]

On Mon, Apr 12, 2021 at 02:09:10PM +0100, Marc Zyngier wrote:
> On Mon, 12 Apr 2021 13:46:37 +0100,
> Catalin Marinas <catalin.marinas at arm.com> wrote:
> > 
> > On Mon, Apr 12, 2021 at 08:08:23PM +0800, yangwendong wrote:
> > > Recently, a new feature of WFE with timeouts has been added to ARMv8.
> > > I have some doubts about the application scenarios of this feature.
> > > 
> > > 1) Arm spec said that WFE or WFET can be used in spinlock. Since the
> > > thread using spinlock can't be sleep, if we use the wfet instruction, we
> > > can do nothing but wait when timeout,  so what's the difference between
> > > the two instructions in this scenario?
> > 
> > Not much point in using it it in a classic spinlock, unless you have
> > some specific implementation that's supposed to time out.
> > 
> > Note that we already enabled the event stream in Linux so that an event
> > is generated at 100KHz waking up any WFE. One reason we had for this was
> > some hardware errata where events between clusters were not generated.
> > Another was some small delays required in in certain user programs
> > without going through a kernel syscall, though not sure anyone's
> > actually using it.
> > 
> > > 2) Are there any other special scenarios where using wfet instructions
> > > can be beneficial ?
> > 
> > In the kernel we could replace our udelay loop with WFIT for example
> > (not WFET because of the event stream). As for user, we can expose a
> > HWCAP but it's up to user libraries to make use of it.
> 
> Note that since c219bc4e9205K ("arm64: Trap WFI executed in
> userspace"), we actively prevent WFI from being used in userspace, and
> I would expect WFIT to be given the same treatment. It otherwise is a
> precise tool for userspace to synchronise against kernel events.

I agree. I only thought about using it in the kernel as a simpler
udelay(). The user should not attempt WFI/WFIT.

Now, if KVM traps WFI/WFIT as well, maybe we should not bother with
udelay() in the kernel either.

-- 
Catalin