KVM: x86: implement PVCLOCK_TSC_STABLE_BIT pvclock flag

KVM added a global variable to guarantee monotonicity in the guest.
One of the reasons for that is that the time between

	1. ktime_get_ts(&timespec);
	2. rdtscll(tsc);

Is variable. That is, given a host with stable TSC, suppose that
two VCPUs read the same time via ktime_get_ts() above.

The time required to execute 2. is not the same on those two instances
executing in different VCPUS (cache misses, interrupts...).

If the TSC value that is used by the host to interpolate when
calculating the monotonic time is the same value used to calculate
the tsc_timestamp value stored in the pvclock data structure, and
a single <system_timestamp, tsc_timestamp> tuple is visible to all
vcpus simultaneously, this problem disappears. See comment on top
of pvclock_update_vm_gtod_copy for details.

Monotonicity is then guaranteed by synchronicity of the host TSCs
and guest TSCs.

Set TSC stable pvclock flag in that case, allowing the guest to read
clock from userspace.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index d60535adec98..32f0e4a063b7 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -22,6 +22,8 @@
 #include <linux/kvm_para.h>
 #include <linux/kvm_types.h>
 #include <linux/perf_event.h>
+#include <linux/pvclock_gtod.h>
+#include <linux/clocksource.h>
 
 #include <asm/pvclock-abi.h>
 #include <asm/desc.h>
@@ -560,6 +562,11 @@ struct kvm_arch {
 	u64 cur_tsc_offset;
 	u8  cur_tsc_generation;
 
+	spinlock_t pvclock_gtod_sync_lock;
+	bool use_master_clock;
+	u64 master_kernel_ns;
+	cycle_t master_cycle_now;
+
 	struct kvm_xen_hvm_config xen_hvm_config;
 
 	/* fields used by HYPER-V emulation */