linux-kernel - Re: [PATCH v4 00/10] make L2's kvm-clock stable, get rid of pvclock_gtod

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Date:   Thu, 24 Aug 2017 10:00:43 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     John Stultz <john.stultz@...aro.org>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        kvm list <kvm@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        lkml <linux-kernel@...r.kernel.org>, x86@...nel.org,
        rkagan@...tuozzo.com, den@...tuozzo.com,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 00/10] make L2's kvm-clock stable, get rid of
 pvclock_gtod_copy in KVM

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