kvmclock: allow stable sched clock

kvmclock provides the behaviour sched_clock users expect. Mark it as stable allowing nohz_full in guests. See individual patches for more details.

Setting sched clock stable for kvmclock causes the printk timestamps to not start from zero, which is different from baremetal and can possibly break userspace. Add a flag to indicate that hypervisor sets clock base at zero when kvmclock is initialized.

---
 arch/x86/include/asm/pvclock-abi.h |    1 +
 1 file changed, 1 insertion(+)

Index: kvm/arch/x86/include/asm/pvclock-abi.h
===================================================================
@@ -41,5 +41,6 @@
 
 #define PVCLOCK_TSC_STABLE_BIT	(1 << 0)
 #define PVCLOCK_GUEST_STOPPED	(1 << 1)
+#define PVCLOCK_COUNTS_FROM_ZERO (1 << 2)
 #endif /* __ASSEMBLY__ */
 #endif /* _ASM_X86_PVCLOCK_ABI_H */

From: Luiz Capitulino <[email protected]>

If you try to enable NOHZ_FULL on a guest today, you'll get the following error when the guest tries to deactivate the scheduler tick:

 WARNING: CPU: 3 PID: 2182 at kernel/time/tick-sched.c:192 can_stop_full_tick+0xb9/0x290()
 NO_HZ FULL will not work with unstable sched clock
 CPU: 3 PID: 2182 Comm: kworker/3:1 Not tainted 4.0.0-10545-gb9bb6fb #204
 Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
 Workqueue: events flush_to_ldisc
  ffffffff8162a0c7 ffff88011f583e88 ffffffff814e6ba0 0000000000000002
  ffff88011f583ed8 ffff88011f583ec8 ffffffff8104d095 ffff88011f583eb8
  0000000000000000 0000000000000003 0000000000000001 0000000000000001
 Call Trace:
  <IRQ>  [<ffffffff814e6ba0>] dump_stack+0x4f/0x7b
  [<ffffffff8104d095>] warn_slowpath_common+0x85/0xc0
  [<ffffffff8104d146>] warn_slowpath_fmt+0x46/0x50
  [<ffffffff810bd2a9>] can_stop_full_tick+0xb9/0x290
  [<ffffffff810bd9ed>] tick_nohz_irq_exit+0x8d/0xb0
  [<ffffffff810511c5>] irq_exit+0xc5/0x130
  [<ffffffff814f180a>] smp_apic_timer_interrupt+0x4a/0x60
  [<ffffffff814eff5e>] apic_timer_interrupt+0x6e/0x80
  <EOI>  [<ffffffff814ee5d1>] ? _raw_spin_unlock_irqrestore+0x31/0x60
  [<ffffffff8108bbc8>] __wake_up+0x48/0x60
  [<ffffffff8134836c>] n_tty_receive_buf_common+0x49c/0xba0
  [<ffffffff8134a6bf>] ? tty_ldisc_ref+0x1f/0x70
  [<ffffffff81348a84>] n_tty_receive_buf2+0x14/0x20
  [<ffffffff8134b390>] flush_to_ldisc+0xe0/0x120
  [<ffffffff81064d05>] process_one_work+0x1d5/0x540
  [<ffffffff81064c81>] ? process_one_work+0x151/0x540
  [<ffffffff81065191>] worker_thread+0x121/0x470
  [<ffffffff81065070>] ? process_one_work+0x540/0x540
  [<ffffffff8106b4df>] kthread+0xef/0x110
  [<ffffffff8106b3f0>] ? __kthread_parkme+0xa0/0xa0
  [<ffffffff814ef4f2>] ret_from_fork+0x42/0x70
  [<ffffffff8106b3f0>] ? __kthread_parkme+0xa0/0xa0
 ---[ end trace 06e3507544a38866 ]---

However, it turns out that kvmclock does provide a stable sched_clock callback. So, let the scheduler know this which in turn makes NOHZ_FULL work in the guest.

---
 arch/x86/kernel/kvmclock.c |   17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

Index: kvm/arch/x86/kernel/kvmclock.c
===================================================================
@@ -24,6 +24,7 @@
 #include <linux/percpu.h>
 #include <linux/hardirq.h>
 #include <linux/memblock.h>
+#include <linux/sched.h>
 
 #include <asm/x86_init.h>
 #include <asm/reboot.h>

@@ -217,8 +218,10 @@
 
 void __init kvmclock_init(void)
 {
+	struct pvclock_vcpu_time_info *vcpu_time;
 	unsigned long mem;
-	int size;
+	int size, cpu;
+	u8 flags;
 
 	size = PAGE_ALIGN(sizeof(struct pvclock_vsyscall_time_info)*NR_CPUS);
 

@@ -263,8 +266,18 @@
 	clocksource_register_hz(&kvm_clock, NSEC_PER_SEC);
 	pv_info.name = "KVM";
 
+	flags = PVCLOCK_COUNTS_FROM_ZERO;
 	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))
-		pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);
+		flags |= PVCLOCK_TSC_STABLE_BIT;
+
+	pvclock_set_flags(flags);
+
+	cpu = get_cpu();
+	vcpu_time = &hv_clock[cpu].pvti;
+	flags = pvclock_read_flags(vcpu_time);
+	if (flags & PVCLOCK_COUNTS_FROM_ZERO)
+		set_sched_clock_stable();
+	put_cpu();
 }
 
 int __init kvm_setup_vsyscall_timeinfo(void)

Initialize kvmclock base, on kvmclock system MSR write time, so that the guest sees kvmclock counting from zero.

This matches baremetal behaviour when kvmclock in guest sets sched clock stable.

---
 arch/x86/kvm/x86.c |    5 +++++
 1 file changed, 5 insertions(+)

Index: kvm/arch/x86/kvm/x86.c
===================================================================
@@ -1703,6 +1703,8 @@
 	/* If the host uses TSC clocksource, then it is stable */
 	if (use_master_clock)
 		pvclock_flags |= PVCLOCK_TSC_STABLE_BIT;
+	if (ka->kvmclk_counts_from_zero)
+		pvclock_flags |= PVCLOCK_COUNTS_FROM_ZERO;
 
 	vcpu->hv_clock.flags = pvclock_flags;
 

@@ -2282,6 +2284,9 @@
 					&vcpu->requests);
 
 			ka->boot_vcpu_runs_old_kvmclock = tmp;
+
+			ka->kvmclock_offset = -get_kernel_ns();
+			ka->kvmclk_counts_from_zero = true;
 		}
 
 		vcpu->arch.time = data;