linux-kernel - Re: [RFC][PATCH] KVM: Remove HIGH_RES

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Message-ID: <20240905161926.186090-2-pbonzini@redhat.com>
Date: Thu,  5 Sep 2024 12:19:27 -0400
From: Paolo Bonzini <pbonzini@...hat.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	kvm@...r.kernel.org,
	Sean Christopherson <seanjc@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	x86@...nel.org,
	Joel Fernandes <joel@...lfernandes.org>,
	Suleiman Souhlal <ssouhlal@...ebsd.org>,
	Vineeth Pillai <vineeth@...byteword.org>,
	Borislav Petkov <bp@...en8.de>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC][PATCH] KVM: Remove HIGH_RES_TIMERS dependency

> Commit 92b5265d38f6a ("KVM: Depend on HIGH_RES_TIMERS") added a dependency
> to high resolution timers with the comment:
> 
>     KVM lapic timer and tsc deadline timer based on hrtimer,
>     setting a leftmost node to rb tree and then do hrtimer reprogram.
>     If hrtimer not configured as high resolution, hrtimer_enqueue_reprogram
>     do nothing and then make kvm lapic timer and tsc deadline timer fail.
> 
> That was back in 2012, where hrtimer_start_range_ns() would do the
> reprogramming with hrtimer_enqueue_reprogram(). But as that was a nop with
> high resolution timers disabled, this did not work. But a lot has changed
> in the last 12 years.
> 
> For example, commit 49a2a07514a3a ("hrtimer: Kick lowres dynticks targets on
> timer enqueue") modifies __hrtimer_start_range_ns() to work with low res
> timers. There's been lots of other changes that make low res work.
> 
> I added this change to my main server that runs all my VMs (my mail
> server, my web server, my ssh server) and disabled HIGH_RES_TIMERS and the
> system has been running just fine for over a month.
> 
> ChromeOS has tested this before as well, and it hasn't seen any issues with
> running KVM with high res timers disabled.
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Queued, thanks.

Paolo