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Re: [RFC PATCH v1 2/2] s390x/kvm: Pass SIGP Stop flags

From: David Hildenbrand
Subject: Re: [RFC PATCH v1 2/2] s390x/kvm: Pass SIGP Stop flags
Date: Mon, 11 Oct 2021 11:21:29 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

On 11.10.21 10:40, Christian Borntraeger wrote:

Am 11.10.21 um 09:09 schrieb David Hildenbrand:
On 08.10.21 22:38, Eric Farman wrote:
When building a Stop IRQ to pass to KVM, we should incorporate
the flags if handling the SIGP Stop and Store Status order.
With that, KVM can reject other orders that are submitted for
the same CPU while the operation is fully processed.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
   target/s390x/kvm/kvm.c | 4 ++++
   1 file changed, 4 insertions(+)

diff --git a/target/s390x/kvm/kvm.c b/target/s390x/kvm/kvm.c
index 5b1fdb55c4..701b9ddc88 100644
--- a/target/s390x/kvm/kvm.c
+++ b/target/s390x/kvm/kvm.c
@@ -2555,6 +2555,10 @@ void kvm_s390_stop_interrupt(S390CPU *cpu)
           .type = KVM_S390_SIGP_STOP,
+    if (cpu->env.sigp_order == SIGP_STOP_STORE_STATUS) {
+        irq.u.stop.flags = KVM_S390_STOP_FLAG_STORE_STATUS;
+    }

KVM_S390_STOP_FLAG_STORE_STATUS tells KVM to perform the store status as well 
... is that really what we want?
At least it should not hurt I guess. QEMU then does it again?

The thing is, that before we officially completed the action in user space (and let other SIGP actions actually succeed in user space on the CPU), the target CPU will be reported as !busy in the kernel already. And before we even inject the stop interrupt, the CPU will be detected as !busy in the kernel. I guess it will fix some cases where we poll via SENSE if the stop and store happened, because the store *did* happen in the kernel and we'll simply store again in user space.

However, I wonder if we want to handle it more generically: Properly flag a CPU as busy for SIGP when we start processing the order until we completed processing the order. That would allow to handle other SIGP operations in user space cleanly, without any chance for races with SENSE code running in the kernel.


David / dhildenb

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