[PATCH v7 14/36] KVM: x86: Improve synchronization in kvm_synchronize_tsc()

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

When synchronizing to an existing TSC (either by explicitly writing
zero, or the legacy hack where the TSC is written within one second's
worth of the previously written TSC), the last_tsc_write and
last_tsc_nsec values were being misrecorded by __kvm_synchronize_tsc().
The *unsynchronized* value of the TSC (perhaps even zero) was being
recorded, along with the current time at which kvm_synchronize_tsc()
was called. This could cause *subsequent* writes to fail to synchronize
correctly.

Fix that by resetting {data, ns} to the previous values before passing
them to __kvm_synchronize_tsc() when synchronization is detected.
Except in the case where the TSC is unstable and *has* to be synthesised
from the host clock, in which case attempt to create a nsec/tsc pair
which is on the correct line.

Furthermore, there were *three* different TSC reads used for calculating
the "current" time, all slightly different from each other. Fix that by
using kvm_get_time_and_clockread() where possible and using the same
host_tsc value in all cases.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Reviewed-by: Paul Durrant <paul@xxxxxxx>
---
 arch/x86/kvm/x86.c | 36 +++++++++++++++++++++++++++---------
 1 file changed, 27 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c4fd2367686c..812657f7c984 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -209,6 +209,10 @@ module_param(pi_inject_timer, bint, 0644);
 static bool __read_mostly mitigate_smt_rsb;
 module_param(mitigate_smt_rsb, bool, 0444);
 
+#ifdef CONFIG_X86_64
+static bool kvm_get_time_and_clockread(s64 *kernel_ns, u64 *tsc_timestamp);
+#endif
+
 #define KVM_SUPPORTED_XCR0     (XFEATURE_MASK_FP | XFEATURE_MASK_SSE \
                                | XFEATURE_MASK_YMM | XFEATURE_MASK_BNDREGS \
                                | XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
@@ -1314,14 +1318,23 @@ void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 
*user_value)
 {
        u64 data = user_value ? *user_value : 0;
        struct kvm *kvm = vcpu->kvm;
-       u64 offset, ns, elapsed;
+       u64 offset, host_tsc, elapsed;
+       s64 ns;
        unsigned long flags;
        bool matched = false;
        bool synchronizing = false;
 
        raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
-       offset = kvm_compute_l1_tsc_offset(vcpu, rdtsc(), data);
-       ns = get_kvmclock_base_ns();
+
+#ifdef CONFIG_X86_64
+       if (!kvm_get_time_and_clockread(&ns, &host_tsc))
+#endif
+       {
+               host_tsc = rdtsc();
+               ns = get_kvmclock_base_ns();
+       }
+
+       offset = kvm_compute_l1_tsc_offset(vcpu, host_tsc, data);
        elapsed = ns - kvm->arch.last_tsc_nsec;
 
        if (vcpu->arch.virtual_tsc_khz) {
@@ -1364,13 +1377,18 @@ void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 
*user_value)
          */
        if (synchronizing &&
            vcpu->arch.virtual_tsc_khz == kvm->arch.last_tsc_khz) {
-               if (!kvm_check_tsc_unstable()) {
+               /*
+                * If synchronizing, advance the reference point to "now"
+                * so the matching window slides forward with each vCPU.
+                */
+               u64 delta = nsec_to_cycles(vcpu, elapsed);
+
+               data = kvm->arch.cur_tsc_write + delta;
+
+               if (!kvm_check_tsc_unstable())
                        offset = kvm->arch.cur_tsc_offset;
-               } else {
-                       u64 delta = nsec_to_cycles(vcpu, elapsed);
-                       data += delta;
-                       offset = kvm_compute_l1_tsc_offset(vcpu, rdtsc(), data);
-               }
+               else
+                       offset = kvm_compute_l1_tsc_offset(vcpu, host_tsc, 
data);
                matched = true;
        }
 
-- 
2.55.0