[PATCH v7 06/36] KVM: x86: Drop CPU pinning in get_kvmclock()

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

The master clock branch of get_kvmclock() pinned the task to a CPU so
that the rdtsc() and the __this_cpu_read(cpu_tsc_khz) would agree. But
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.

Given that, the open-coded CONSTANT_TSC || cpu_tsc_khz check is
redundant (with CONSTANT_TSC, cpu_tsc_khz is tsc_khz on every CPU and
KVM does not even register the cpufreq notifier), and both the TSC read
and get_cpu_tsc_khz() are CPU-independent. Drop the condition and the
get_cpu()/put_cpu() pinning.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1a598e99852c..590a7ba723f7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1659,12 +1659,8 @@ static void get_kvmclock(struct kvm *kvm, struct 
kvm_clock_data *data)
        do {
                seq = read_seqcount_begin(&ka->pvclock_sc);
 
-               /* both __this_cpu_read() and rdtsc() should be on the same cpu 
*/
-               get_cpu();
-
                data->flags = 0;
-               if (ka->use_master_clock &&
-                   (static_cpu_has(X86_FEATURE_CONSTANT_TSC) || 
__this_cpu_read(cpu_tsc_khz))) {
+               if (ka->use_master_clock) {
 #ifdef CONFIG_X86_64
                        struct timespec64 ts;
 
@@ -1685,8 +1681,6 @@ static void get_kvmclock(struct kvm *kvm, struct 
kvm_clock_data *data)
                } else {
                        data->clock = get_kvmclock_base_ns() + 
ka->kvmclock_offset;
                }
-
-               put_cpu();
        } while (read_seqcount_retry(&ka->pvclock_sc, seq));
 }
 
-- 
2.55.0