Re: [PATCH RFC 4/6] KVM: s390: consider epoch index on TOD clock syncs

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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




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