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

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Message-ID: <20240821095127.45d17b19@gandalf.local.home>
Date: Wed, 21 Aug 2024 09:51:27 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>, Paolo
 Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
 "x86@...nel.org" <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: [RFC][PATCH] KVM: Remove HIGH_RES_TIMERS dependency

From: Steven Rostedt <rostedt@...dmis.org>

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>
---
 arch/x86/kvm/Kconfig | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 472a1537b7a9..c65127e796a9 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -19,7 +19,6 @@ if VIRTUALIZATION

 config KVM
 	tristate "Kernel-based Virtual Machine (KVM) support"
-	depends on HIGH_RES_TIMERS
 	depends on X86_LOCAL_APIC
 	select KVM_COMMON
 	select KVM_GENERIC_MMU_NOTIFIER
-- 
2.43.0