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Subject[PATCH v2 6/7] x86/kvm: pass stable clocksource to guests when running nested on Hyper-V
Date
Currently, KVM is able to work in 'masterclock' mode passing
PVCLOCK_TSC_STABLE_BIT to guests when the clocksource we use on the host
is TSC. When running nested on Hyper-V we normally use a different one:
TSC page which is resistant to TSC frequency changes on event like L1
migration. Add support for it in KVM.

The only non-trivial change in the patch is in vgettsc(): when updating
our gtod copy we now need to get both the clockread and tsc value.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
arch/x86/kvm/x86.c | 93 +++++++++++++++++++++++++++++++++++++++---------------
1 file changed, 68 insertions(+), 25 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index faf843c9b916..843a8596f57e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -67,6 +67,7 @@
#include <asm/pvclock.h>
#include <asm/div64.h>
#include <asm/irq_remapping.h>
+#include <asm/mshyperv.h>

#define CREATE_TRACE_POINTS
#include "trace.h"
@@ -1377,6 +1378,11 @@ static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns)
return tsc;
}

+static inline int gtod_is_based_on_tsc(int mode)
+{
+ return mode == VCLOCK_TSC || mode == VCLOCK_HVCLOCK;
+}
+
static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
{
#ifdef CONFIG_X86_64
@@ -1396,7 +1402,7 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
* perform request to enable masterclock.
*/
if (ka->use_master_clock ||
- (gtod->clock.vclock_mode == VCLOCK_TSC && vcpus_matched))
+ (gtod_is_based_on_tsc(gtod->clock.vclock_mode) && vcpus_matched))
kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);

trace_kvm_track_tsc(vcpu->vcpu_id, ka->nr_vcpus_matched_tsc,
@@ -1459,6 +1465,19 @@ static void kvm_vcpu_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
vcpu->arch.tsc_offset = offset;
}

+static inline bool kvm_check_tsc_unstable(void)
+{
+#ifdef CONFIG_X86_64
+ /*
+ * TSC is marked unstable when we're running on Hyper-V,
+ * 'TSC page' clocksource is good.
+ */
+ if (pvclock_gtod_data.clock.vclock_mode == VCLOCK_HVCLOCK)
+ return false;
+#endif
+ return check_tsc_unstable();
+}
+
void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
{
struct kvm *kvm = vcpu->kvm;
@@ -1504,7 +1523,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
*/
if (synchronizing &&
vcpu->arch.virtual_tsc_khz == kvm->arch.last_tsc_khz) {
- if (!check_tsc_unstable()) {
+ if (!kvm_check_tsc_unstable()) {
offset = kvm->arch.cur_tsc_offset;
pr_debug("kvm: matched tsc offset for %llu\n", data);
} else {
@@ -1604,18 +1623,43 @@ static u64 read_tsc(void)
return last;
}

-static inline u64 vgettsc(u64 *cycle_now)
+static inline u64 vgettsc(u64 *tsc_timestamp, int *mode)
{
long v;
struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
+ u64 tsc_pg_val;
+
+ switch (gtod->clock.vclock_mode) {
+ case VCLOCK_HVCLOCK:
+ tsc_pg_val = hv_read_tsc_page_tsc(hv_get_tsc_page(),
+ tsc_timestamp);
+ if (tsc_pg_val != U64_MAX) {
+ /* TSC page valid */
+ *mode = VCLOCK_HVCLOCK;
+ v = (tsc_pg_val - gtod->clock.cycle_last) &
+ gtod->clock.mask;
+ } else {
+ /* TSC page invalid */
+ *mode = VCLOCK_NONE;
+ }
+ break;
+ case VCLOCK_TSC:
+ *mode = VCLOCK_TSC;
+ *tsc_timestamp = read_tsc();
+ v = (*tsc_timestamp - gtod->clock.cycle_last) &
+ gtod->clock.mask;
+ break;
+ default:
+ *mode = VCLOCK_NONE;
+ }

- *cycle_now = read_tsc();
+ if (*mode == VCLOCK_NONE)
+ *tsc_timestamp = v = 0;

- v = (*cycle_now - gtod->clock.cycle_last) & gtod->clock.mask;
return v * gtod->clock.mult;
}

-static int do_monotonic_boot(s64 *t, u64 *cycle_now)
+static int do_monotonic_boot(s64 *t, u64 *tsc_timestamp)
{
struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
unsigned long seq;
@@ -1624,9 +1668,8 @@ static int do_monotonic_boot(s64 *t, u64 *cycle_now)

do {
seq = read_seqcount_begin(&gtod->seq);
- mode = gtod->clock.vclock_mode;
ns = gtod->nsec_base;
- ns += vgettsc(cycle_now);
+ ns += vgettsc(tsc_timestamp, &mode);
ns >>= gtod->clock.shift;
ns += gtod->boot_ns;
} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
@@ -1635,7 +1678,7 @@ static int do_monotonic_boot(s64 *t, u64 *cycle_now)
return mode;
}

-static int do_realtime(struct timespec *ts, u64 *cycle_now)
+static int do_realtime(struct timespec *ts, u64 *tsc_timestamp)
{
struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
unsigned long seq;
@@ -1644,10 +1687,9 @@ static int do_realtime(struct timespec *ts, u64 *cycle_now)

do {
seq = read_seqcount_begin(&gtod->seq);
- mode = gtod->clock.vclock_mode;
ts->tv_sec = gtod->wall_time_sec;
ns = gtod->nsec_base;
- ns += vgettsc(cycle_now);
+ ns += vgettsc(tsc_timestamp, &mode);
ns >>= gtod->clock.shift;
} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));

@@ -1657,25 +1699,26 @@ static int do_realtime(struct timespec *ts, u64 *cycle_now)
return mode;
}

-/* returns true if host is using tsc clocksource */
-static bool kvm_get_time_and_clockread(s64 *kernel_ns, u64 *cycle_now)
+/* returns true if host is using TSC based clocksource */
+static bool kvm_get_time_and_clockread(s64 *kernel_ns, u64 *tsc_timestamp)
{
/* checked again under seqlock below */
- if (pvclock_gtod_data.clock.vclock_mode != VCLOCK_TSC)
+ if (!gtod_is_based_on_tsc(pvclock_gtod_data.clock.vclock_mode))
return false;

- return do_monotonic_boot(kernel_ns, cycle_now) == VCLOCK_TSC;
+ return gtod_is_based_on_tsc(do_monotonic_boot(kernel_ns,
+ tsc_timestamp));
}

-/* returns true if host is using tsc clocksource */
+/* returns true if host is using TSC based clocksource */
static bool kvm_get_walltime_and_clockread(struct timespec *ts,
- u64 *cycle_now)
+ u64 *tsc_timestamp)
{
/* checked again under seqlock below */
- if (pvclock_gtod_data.clock.vclock_mode != VCLOCK_TSC)
+ if (!gtod_is_based_on_tsc(pvclock_gtod_data.clock.vclock_mode))
return false;

- return do_realtime(ts, cycle_now) == VCLOCK_TSC;
+ return gtod_is_based_on_tsc(do_realtime(ts, tsc_timestamp));
}
#endif

@@ -2869,13 +2912,13 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
}

- if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
+ if (unlikely(vcpu->cpu != cpu) || kvm_check_tsc_unstable()) {
s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
rdtsc() - vcpu->arch.last_host_tsc;
if (tsc_delta < 0)
mark_tsc_unstable("KVM discovered backwards TSC");

- if (check_tsc_unstable()) {
+ if (kvm_check_tsc_unstable()) {
u64 offset = kvm_compute_tsc_offset(vcpu,
vcpu->arch.last_guest_tsc);
kvm_vcpu_write_tsc_offset(vcpu, offset);
@@ -6110,9 +6153,9 @@ static int pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
update_pvclock_gtod(tk);

/* disable master clock if host does not trust, or does not
- * use, TSC clocksource
+ * use, TSC based clocksource.
*/
- if (gtod->clock.vclock_mode != VCLOCK_TSC &&
+ if (!gtod_is_based_on_tsc(gtod->clock.vclock_mode) &&
atomic_read(&kvm_guest_has_master_clock) != 0)
queue_work(system_long_wq, &pvclock_gtod_work);

@@ -7745,7 +7788,7 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm,
{
struct kvm_vcpu *vcpu;

- if (check_tsc_unstable() && atomic_read(&kvm->online_vcpus) != 0)
+ if (kvm_check_tsc_unstable() && atomic_read(&kvm->online_vcpus) != 0)
printk_once(KERN_WARNING
"kvm: SMP vm created on host with unstable TSC; "
"guest TSC will not be reliable\n");
@@ -7902,7 +7945,7 @@ int kvm_arch_hardware_enable(void)
return ret;

local_tsc = rdtsc();
- stable = !check_tsc_unstable();
+ stable = !kvm_check_tsc_unstable();
list_for_each_entry(kvm, &vm_list, vm_list) {
kvm_for_each_vcpu(i, vcpu, kvm) {
if (!stable && vcpu->cpu == smp_processor_id())
--
2.14.3
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