From: Steven Rostedt Date: Wed, 21 Aug 2024 13:51:27 +0000 (-0400) Subject: KVM: Remove HIGH_RES_TIMERS dependency X-Git-Tag: for-6.12/io_uring-discard-20240913~24^2 X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=59cbd4eea48fdbc68fc17a29ad71188fea74b28b;p=linux-2.6-block.git 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. ChromeOS has tested this before as well, and it hasn't seen any issues with running KVM with high res timers disabled. There could be problems, especially at low HZ, for guests that do not support kvmclock and rely on precise delivery of periodic timers to keep their clock running. This can be the APIC timer (provided by the kernel), the RTC (provided by userspace), or the i8254 (choice of kernel/userspace). These guests are few and far between these days, and in the case of the APIC timer + Intel hosts we can use the preemption timer (which is TSC-based and has better latency _and_ accuracy). In KVM, only x86 is requiring CONFIG_HIGH_RES_TIMERS; perhaps a "depends on HIGH_RES_TIMERS || EXPERT" could be added to virt/kvm, or a pr_warn could be added to kvm_init if HIGH_RES_TIMERS are not enabled. But in general, it seems that there must be other code in the kernel (maybe sound/?) that is relying on having high-enough HZ or hrtimers but that's not documented anywhere. Whenever you disable it you probably need to know what you're doing and what your workload is; so the dependency is not particularly interesting, and we can just remove it. Signed-off-by: Steven Rostedt (Google) Message-ID: <20240821095127.45d17b19@gandalf.local.home> [Added the last two paragraphs to the commit message. - Paolo] Signed-off-by: Paolo Bonzini --- diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig index faed96e33e38..730c2f34d347 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