Re: [kvm-unit-tests PATCH] s390x: Workaround smp stop and store status race

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On 09/02/2021 15.15, Janosch Frank wrote:
KVM and QEMU handle a SIGP stop and store status in two steps:
1) Stop the CPU by injecting a stop request
2) Store when the CPU has left SIE because of the stop request

The problem is that the SIGP order is already considered completed by
KVM/QEMU when step 1 has been performed and not once both have
completed. In addition we currently don't implement the busy CC so a
kernel has no way of knowing that the store has finished other than
checking the location for the store.

This workaround is based on the fact that for a new SIE entry (via the
added smp restart) a stop with the store status has to be finished
first.

Correct handling of this in KVM/QEMU will need some thought and time.

Signed-off-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
---
  s390x/smp.c | 5 ++++-
  1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/s390x/smp.c b/s390x/smp.c
index b0ece491..32f284a2 100644
--- a/s390x/smp.c
+++ b/s390x/smp.c
@@ -102,12 +102,15 @@ static void test_stop_store_status(void)
  	lc->grs_sa[15] = 0;
  	smp_cpu_stop_store_status(1);
  	mb();
+	report(smp_cpu_stopped(1), "cpu stopped");
+	/* For the cpu to be started it should have finished storing */
+	smp_cpu_restart(1);
  	report(lc->prefix_sa == (uint32_t)(uintptr_t)cpu->lowcore, "prefix");
  	report(lc->grs_sa[15], "stack");
-	report(smp_cpu_stopped(1), "cpu stopped");
  	report_prefix_pop();
report_prefix_push("stopped");
+	smp_cpu_stop(1);
  	lc->prefix_sa = 0;
  	lc->grs_sa[15] = 0;
  	smp_cpu_stop_store_status(1);

Thanks, this fixes the flaky test for me!

Tested-by: Thomas Huth <thuth@xxxxxxxxxx>
Reviewed-by: Thomas Huth <thuth@xxxxxxxxxx>





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