[PATCH v5 11/34] KVM: x86: Restructure get_kvmclock()

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

Move get/put_cpu inside the use_master_clock branch since they are only
needed there (for RDTSC and get_cpu_tsc_khz() to be on the same CPU).

Simplify the use_master_clock condition: the open-coded CONSTANT_TSC ||
cpu_tsc_khz check is unnecessary since use_master_clock can only be true
when the TSC is usable.

Wrap the entire use_master_clock block in #ifdef CONFIG_X86_64, since
use_master_clock is never true on 32-bit (host_tsc_clocksource is only
set under CONFIG_X86_64).

When the clock read fails (e.g. clocksource transitioning away from
TSC), fall back to the non-master-clock path (get_kvmclock_base_ns)
rather than proceeding with uninitialised data or spinning in the
seqcount loop.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fce898811fe7..6983a7494fcd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3209,21 +3209,30 @@ 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))) {
 #ifdef CONFIG_X86_64
+               if (ka->use_master_clock) {
                        struct timespec64 ts;
 
+                       /*
+                        * The RDTSC and get_cpu_tsc_khz() must happen on
+                        * the same CPU.
+                        */
+                       get_cpu();
+
                        if (kvm_get_walltime_and_clockread(&ts, 
&data->host_tsc)) {
                                data->realtime = ts.tv_nsec + NSEC_PER_SEC * 
ts.tv_sec;
                                data->flags |= KVM_CLOCK_REALTIME | 
KVM_CLOCK_HOST_TSC;
-                       } else
-#endif
-                       data->host_tsc = rdtsc();
+                       } else {
+                               /*
+                                * Clock read failed (e.g. clocksource is
+                                * transitioning away from TSC). Fall back to
+                                * the non-master-clock path rather than
+                                * spinning.
+                                */
+                               put_cpu();
+                               goto fallback;
+                       }
 
                        data->flags |= KVM_CLOCK_TSC_STABLE;
                        hv_clock.tsc_timestamp = ka->master_cycle_now;
@@ -3232,11 +3241,14 @@ static void get_kvmclock(struct kvm *kvm, struct 
kvm_clock_data *data)
                                           &hv_clock.tsc_shift,
                                           &hv_clock.tsc_to_system_mul);
                        data->clock = __pvclock_read_cycles(&hv_clock, 
data->host_tsc);
-               } else {
+
+                       put_cpu();
+               } else
+#endif
+               {
+fallback:
                        data->clock = get_kvmclock_base_ns() + 
ka->kvmclock_offset;
                }
-
-               put_cpu();
        } while (read_seqcount_retry(&ka->pvclock_sc, seq));
 }
 
-- 
2.54.0