From: David Woodhouse <dwmw@amazon.co.uk>
The upcoming patches will minimize the chances of updating the master clock
data. Unfortunately, this may cause issues in compute_guest_tsc().
Since the KVM clock reference point in master_kernel_ns might actually be
*earlier* than the reference point used for the guest TSC
(vcpu->last_tsc_nsec), this might lead to a negative delta to be passed to
pvclock_scale_delta(). Fix the compute_guest_tsc() function to cope with
negative numbers.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/all/20240522001817.619072-11-dwmw2@infradead.org/
[Dongli: copy relevant code from above link and modify changelog]
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
---
This a partial copy of "[RFC PATCH v3 10/21] KVM: x86: Fix software
TSC upscaling in kvm_update_guest_time()", as Sean suggested, "Please do
this in a separate patch. There's no need to squeeze it in here, and this
change is complex/subtle enough as it is.", and David's authorship is
preserved.
arch/x86/kvm/x86.c | 28 +++++++++++++++++-----------
1 file changed, 17 insertions(+), 11 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 63afdb6bb078..5e7418cfd0af 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2581,10 +2581,19 @@ 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;
+ s64 delta = kernel_ns - vcpu->arch.this_tsc_nsec;
+ u64 tsc = vcpu->arch.this_tsc_write;
+
+ /* pvclock_scale_delta cannot cope with negative deltas */
+ if (delta >= 0)
+ tsc += pvclock_scale_delta(delta,
+ vcpu->arch.virtual_tsc_mult,
+ vcpu->arch.virtual_tsc_shift);
+ else
+ tsc -= pvclock_scale_delta(-delta,
+ vcpu->arch.virtual_tsc_mult,
+ vcpu->arch.virtual_tsc_shift);
+
return tsc;
}
@@ -2595,7 +2604,7 @@ static inline bool gtod_is_based_on_tsc(int mode)
}
#endif
-static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation)
+static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
{
#ifdef CONFIG_X86_64
struct kvm_arch *ka = &vcpu->kvm->arch;
@@ -2612,12 +2621,9 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation)
/*
* Request a masterclock update if the masterclock needs to be toggled
- * on/off, or when starting a new generation and the masterclock is
- * enabled (compute_guest_tsc() requires the masterclock snapshot to be
- * taken _after_ the new generation is created).
+ * on/off.
*/
- if ((ka->use_master_clock && new_generation) ||
- (ka->use_master_clock != use_master_clock))
+ if (ka->use_master_clock != use_master_clock)
kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
trace_kvm_track_tsc(vcpu->vcpu_id, ka->nr_vcpus_matched_tsc,
@@ -2803,7 +2809,7 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
vcpu->arch.this_tsc_nsec = kvm->arch.cur_tsc_nsec;
vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
- kvm_track_tsc_matching(vcpu, !matched);
+ kvm_track_tsc_matching(vcpu);
}
static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 *user_value)
--
2.39.3