Re: [PATCH v4 00/10] make L2's kvm-clock stable, get rid of pvclock_gtod_copy in KVM

Paolo Bonzini
2017-08-24T08:00:43Z

On 23/08/2017 18:02, Paolo Bonzini wrote:
>
> More duct tape would have been just:
>
> - if (pvclock_gtod_data.clock.vclock_mode != VCLOCK_TSC)
> + mode = READ_ONCE(pvclock_gtod_data.clock.vclock_mode);
> + if (mode != VCLOCK_TSC &&
> + (mode != VCLOCK_PVCLOCK || !pvclock_nested_virt_magic())
> return false;
>
> - return do_realtime(ts, cycle_now) == VCLOCK_TSC;
> + switch (mode) {
> + case VCLOCK_TSC:
> + return do_realtime_tsc(ts, cycle_now);
> + case VCLOCK_PVCLOCK:
> + return do_realtime_pvclock(ts, cycle_now);
> + }
>
> Nested virtualization does need a clocksource change notifier on top,
> but we can cross that bridge later. Maybe Denis can post just those
> patches to begin with.

For what it's worth, this is all that's needed (with patches 1-2-3-4-5-7)
to support kvmclock on top of Hyper-V clock. It's trivial.

Even if we could add paravirtualization magic to KVM live migration, we
certainly couldn't do that for other hypervisors.

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 5b882cc0c0e9..3bab935b021a 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -46,10 +46,24 @@ static u64 read_hv_clock_tsc(struct clocksource *arg)
return current_tick;
}

+static bool read_hv_clock_tsc_with_stamp(struct clocksource *arg,
+ u64 *cycles, u64 *cycles_stamp)
+{
+ *cycles = __hv_read_tsc_page(tsc_pg, &cycles_stamp);
+
+ if (*cycles == U64_MAX) {
+ *cycles = rdmsrl(HV_X64_MSR_TIME_REF_COUNT);
+ return false;
+ }
+
+ return true;
+}
+
static struct clocksource hyperv_cs_tsc = {
.name = "hyperv_clocksource_tsc_page",
.rating = 400,
.read = read_hv_clock_tsc,
+ .read_with_stamp = read_hv_clock_tsc_with_stamp,
.mask = CLOCKSOURCE_MASK(64),
.flags = CLOCK_SOURCE_IS_CONTINUOUS,
};
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 2b58c8c1eeaa..5aff66e9fff7 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -176,9 +176,9 @@ void hyperv_cleanup(void);
#endif
#ifdef CONFIG_HYPERV_TSCPAGE
struct ms_hyperv_tsc_page *hv_get_tsc_page(void);
-static inline u64 hv_read_tsc_page(const struct ms_hyperv_tsc_page *tsc_pg)
+static inline u64 __hv_read_tsc_page(const struct ms_hyperv_tsc_page *tsc_pg, u64 *cur_tsc)
{
- u64 scale, offset, cur_tsc;
+ u64 scale, offset;
u32 sequence;

/*
@@ -209,7 +209,7 @@ static inline u64 hv_read_tsc_page(const struct ms_hyperv_tsc_page *tsc_pg)

scale = READ_ONCE(tsc_pg->tsc_scale);
offset = READ_ONCE(tsc_pg->tsc_offset);
- cur_tsc = rdtsc_ordered();
+ *cur_tsc = rdtsc_ordered();

/*
* Make sure we read sequence after we read all other values
@@ -219,9 +219,14 @@ static inline u64 hv_read_tsc_page(const struct ms_hyperv_tsc_page *tsc_pg)

} while (READ_ONCE(tsc_pg->tsc_sequence) != sequence);

- return mul_u64_u64_shr(cur_tsc, scale, 64) + offset;
+ return mul_u64_u64_shr(*cur_tsc, scale, 64) + offset;
}

+static inline u64 hv_read_tsc_page(const struct ms_hyperv_tsc_page *tsc_pg)
+{
+ u64 cur_tsc;
+ return __hv_read_tsc_page(tsc_pg, &cur_tsc);
+}
#else
static inline struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
{

Denis, could you try redoing patch 7 to use the pvclock_gtod_notifier
instead of the new one you're adding, and only send that first part? I
think it's a worthwhile cleanup anyway, so let's start with that.

Paolo