[PATCH v7 10/36] KVM: x86: Fix compute_guest_tsc() to handle negative time deltas

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

The compute_guest_tsc() function computes the guest TSC at a given
kernel_ns timestamp. When the master clock reference point
(master_kernel_ns) is earlier than vcpu->arch.this_tsc_nsec, the delta
is negative. Since pvclock_scale_delta() takes a u64, the negative
value wraps to a huge positive number, producing a wildly wrong result.

Handle negative deltas explicitly by scaling the absolute value of the
delta and applying it to this_tsc_write with the appropriate sign.

This is believed to be unreachable in practice; no path has been
identified which invokes compute_guest_tsc() with a timestamp from
before the vCPU's TSC generation was established. Fix it for
robustness, in the spirit of defence in depth.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f7a9fb69e69d..52c9268007f8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1054,11 +1054,15 @@ static int kvm_set_tsc_khz(struct kvm_vcpu *vcpu, u32 
user_tsc_khz)
 
 static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns)
 {
-       u64 tsc = pvclock_scale_delta(kernel_ns-vcpu->arch.this_tsc_nsec,
-                                     vcpu->arch.virtual_tsc_mult,
-                                     vcpu->arch.virtual_tsc_shift);
-       tsc += vcpu->arch.this_tsc_write;
-       return tsc;
+       s64 delta_ns = kernel_ns - vcpu->arch.this_tsc_nsec;
+       u64 tsc;
+
+       /* Handle negative deltas gracefully (master clock ref may be earlier) 
*/
+       tsc = pvclock_scale_delta(abs(delta_ns),
+                                 vcpu->arch.virtual_tsc_mult,
+                                 vcpu->arch.virtual_tsc_shift);
+
+       return vcpu->arch.this_tsc_write + (delta_ns >= 0 ? tsc : -tsc);
 }
 
 #ifdef CONFIG_X86_64
-- 
2.55.0