linux-kernel - Re: [PATCH v3 1/5] asm-generic: barrier: Add smp_cond_load_relaxed

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Message-Id: <1ccb0011-d2d2-453f-afcd-dd2967bf572a@app.fastmail.com>
Date: Mon, 18 Aug 2025 13:51:28 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Catalin Marinas" <catalin.marinas@....com>
Cc: "Ankur Arora" <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
 Linux-Arch <linux-arch@...r.kernel.org>,
 linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org,
 "Will Deacon" <will@...nel.org>, "Peter Zijlstra" <peterz@...radead.org>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Mark Rutland" <mark.rutland@....com>, harisokn@...zon.com,
 "Christoph Lameter (Ampere)" <cl@...two.org>,
 "Alexei Starovoitov" <ast@...nel.org>,
 "Kumar Kartikeya Dwivedi" <memxor@...il.com>, zhenglifeng1@...wei.com,
 xueshuai@...ux.alibaba.com, "Joao Martins" <joao.m.martins@...cle.com>,
 "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
 "Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
 "Rafael J . Wysocki" <rafael@...nel.org>,
 "Daniel Lezcano" <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v3 1/5] asm-generic: barrier: Add smp_cond_load_relaxed_timewait()

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.

> 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.

     Arnd