On 02/07/2018 06:46 AM, David Hildenbrand wrote:
For now, we don't take care of over/underflows. Especially underflows
are critical:
Assume the epoch is currently 0 and we get a sync request for delta=1,
meaning the TOD is moved forward by 1 and we have to fix it up by
subtracting 1 from the epoch. Right now, this will leave the epoch
index untouched, resulting in epoch=-1, epoch_idx=0, which is wrong.
We have to take care of over and underflows, also for the VSIE case. So
let's factor out calculation into a separate function.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
arch/s390/kvm/kvm-s390.c | 32 +++++++++++++++++++++++++++++---
1 file changed, 29 insertions(+), 3 deletions(-)
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index d007b737cd4d..c2b62379049e 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -179,6 +179,28 @@ int kvm_arch_hardware_enable(void)
static void kvm_gmap_notifier(struct gmap *gmap, unsigned long start,
unsigned long end);
+static void kvm_clock_sync_scb(struct kvm_s390_sie_block *scb, u64 delta)
+{
+ u64 delta_idx = 0;
+
+ /*
+ * The TOD jumps by delta, we have to compensate this by adding
+ * -delta to the epoch.
+ */
+ delta = -delta;
+
+ /* sign-extension - we're adding to signed values below */
+ if ((s64)delta < 0)
+ delta_idx = 0xff;
+
+ scb->epoch += delta;
+ if (scb->ecd & ECD_MEF) {
+ scb->epdx += delta_idx;
+ if (scb->epoch < delta)
+ scb->epdx += 1;
+ }
+}
+
Is the sync always a jump forward? Do we need to worry about a borrow
from the epdx in case of underflow?
/*
* This callback is executed during stop_machine(). All CPUs are therefore
* temporarily stopped. In order not to change guest behavior, we have to
@@ -194,13 +216,17 @@ static int kvm_clock_sync(struct notifier_block *notifier, unsigned long val,
unsigned long long *delta = v;
list_for_each_entry(kvm, &vm_list, vm_list) {
- kvm->arch.epoch -= *delta;
kvm_for_each_vcpu(i, vcpu, kvm) {
- vcpu->arch.sie_block->epoch -= *delta;
+ kvm_clock_sync_scb(vcpu->arch.sie_block, *delta);
+ if (i == 0) {
+ kvm->arch.epoch = vcpu->arch.sie_block->epoch;
+ kvm->arch.epdx = vcpu->arch.sie_block->epdx;
Are we safe by setting the kvm epochs to the sie epochs wrt migration?
+ }
if (vcpu->arch.cputm_enabled)
vcpu->arch.cputm_start += *delta;
if (vcpu->arch.vsie_block)
- vcpu->arch.vsie_block->epoch -= *delta;
+ kvm_clock_sync_scb(vcpu->arch.vsie_block,
+ *delta);
}
}
return NOTIFY_OK;
--
- Collin L Walling
