KVM: x86: Drop unnecessary CPU pinning when computing/getting kvmclock

When computing the current kvmclock value, don't pin the task to the
current CPU for the entire duration of the master clock path, as the
CPU pinning was never about ensuring rdtsc() and cpu_tsc_khz would
agree.  As pointed out by David, ka->use_master_clock can only be true
when the host clocksource is TSC based, which in turn requires a stable,
constant and synchronised TSC across all CPUs.

The CPU pinning was added in commit e2c2206a1899 ("KVM: x86: Fix potential
preemption when get the current kvmclock timestamp") purely in response to
a CONFIG_DEBUG_PREEMPT=y bug due to accessing a per-CPU variable with
preemption enabled.  Despite what the comment would suggest, including
rdtsc() in the {get,put}_cpu() section was opportunistic.  In fact, Paolo
even said exactly that when suggesting that KVM guarantee the rdtsc() would
execute on the same CPU[*]:

 : Also, rdtsc() should really be on the same CPU as __this_cpu_read.  We
 : know it's not really really necessary because the master clock is
 : active, but since we need a get_cpu/put_cpu pair, better be clean.

Nothing has changed in the last ~9 years, i.e. the rdtsc() still *should*
be on the same CPU, but super strictly speaking, all will be fine if the
task is migrated between grabbing the frequency and doing rdtsc().
Dropping the CPU pinning will allow dropping the rdtsc() entirely without
having to resort to a large "rewrite get_kvmclock()" patch.

Opportunistically add a comment to explain why KVM needs to snapshot the
frequency, because that _is_ a hard requirement to avoid reintroducing the
bug fixed by commit e70b57a6ce4e ("KVM: X86: Fix softlockup when get the
current kvmclock")

Link: https://lore.kernel.org/all/ae8de642-8f14-a70a-1fab-57e2c4093cd5@redhat.com [*]
Suggested-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://patch.msgid.link/20260826213303.914988-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 9 insertions, 6 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 113c9c564ad3..b26699cf302d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1657,13 +1657,18 @@ static void __get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
 {
 	struct kvm_arch *ka = &kvm->arch;
 	struct pvclock_vcpu_time_info hv_clock;
+	u64 tsc_hz;
 
-	/* both __this_cpu_read() and rdtsc() should be on the same cpu */
+	/*
+	 * Snapshot and validate the TSC frequency as kvmclock_cpu_down_prep()
+	 * zeros the per-CPU value when a CPU is going offline.
+	 */
 	get_cpu();
+	tsc_hz = (u64)get_cpu_tsc_khz() * HZ_PER_KHZ;
+	put_cpu();
 
 	data->flags = 0;
-	if (ka->use_master_clock &&
-	    (cpu_feature_enabled(X86_FEATURE_CONSTANT_TSC) || __this_cpu_read(cpu_tsc_khz))) {
+	if (ka->use_master_clock && tsc_hz) {
 #ifdef CONFIG_X86_64
 		struct timespec64 ts;
 
@@ -1677,15 +1682,13 @@ static void __get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
 		data->flags |= KVM_CLOCK_TSC_STABLE;
 		hv_clock.tsc_timestamp = ka->master_cycle_now;
 		hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
-		kvm_get_time_scale(NSEC_PER_SEC, get_cpu_tsc_khz() * 1000LL,
+		kvm_get_time_scale(NSEC_PER_SEC,  tsc_hz,
 				   &hv_clock.tsc_shift,
 				   &hv_clock.tsc_to_system_mul);
 		data->clock = __pvclock_read_cycles(&hv_clock, data->host_tsc);
 	} else {
 		data->clock = get_kvmclock_base_ns() + ka->kvmclock_offset;
 	}
-
-	put_cpu();
 }
 
 static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)