Re: [PATCH RFC V6 1/5] kvm hypervisor : Add a hypercall to KVM hypervisor to support pv-ticketlocks

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Thanks Avi, for the review.

On 04/29/2012 06:55 PM, Avi Kivity wrote:
On 04/23/2012 12:59 PM, Raghavendra K T wrote:
From: Srivatsa Vaddagiri<vatsa@xxxxxxxxxxxxxxxxxx>

KVM_HC_KICK_CPU allows the calling vcpu to kick another vcpu out of halt state.

The presence of these hypercalls is indicated to guest via
KVM_FEATURE_PV_UNHALT/KVM_CAP_PV_UNHALT.

  #endif
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e216ba0..dad475b 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -481,6 +481,10 @@ struct kvm_vcpu_arch {
  		u64 length;
  		u64 status;
  	} osvw;
+	/* pv related host specific info */
+	struct {
+		int pv_unhalted;
+	} pv;
  };

'bool'.  Or maybe push into vcpu->requests.

Ok. I think you meant
+	struct {
+		bool pv_unhalted;
+	} pv;

and as discussed in old series (V4), cleaner implementation having
vcpu request, would still need a flag to prevent vcpu hang, so back to
having one flag.



diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4044ce0..7fc9be6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2147,6 +2147,7 @@ int kvm_dev_ioctl_check_extension(long ext)
  	case KVM_CAP_ASYNC_PF:
  	case KVM_CAP_GET_TSC_KHZ:
  	case KVM_CAP_PCI_2_3:
+	case KVM_CAP_PV_UNHALT:
  		r = 1;
  		break;
  	case KVM_CAP_COALESCED_MMIO:

Redundant, since we can infer this from KVM_GET_SUPPORTED_CPUID.  But
please indicate this in the documentation.


Ok. will mention that in documentation added for KVM_CAP_PV_UNHALT.


+/*
+ * kvm_pv_kick_cpu_op:  Kick a vcpu.
+ *
+ * @apicid - apicid of vcpu to be kicked.
+ */
+static void kvm_pv_kick_cpu_op(struct kvm *kvm, int apicid)
+{
+	struct kvm_vcpu *vcpu = NULL;
+	int i;
+
+	kvm_for_each_vcpu(i, vcpu, kvm) {
+		if (!kvm_apic_present(vcpu))
+			continue;
+
+		if (kvm_apic_match_dest(vcpu, 0, 0, apicid, 0))
+			break;
+	}
+	if (vcpu) {
+		/*
+		 * Setting unhalt flag here can result in spurious runnable
+		 * state when unhalt reset does not happen in vcpu_block.
+		 * But that is harmless since that should soon result in halt.
+		 */
+		vcpu->arch.pv.pv_unhalted = 1;
+		/* We need everybody see unhalt before vcpu unblocks */
+		smp_mb();

smp_wmb().


Done.

+		kvm_vcpu_kick(vcpu);
+	}
+}
+

  /*
   * hypercalls use architecture specific
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 42b7393..edf56d4 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1500,6 +1500,14 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
  		prepare_to_wait(&vcpu->wq,&wait, TASK_INTERRUPTIBLE);

  		if (kvm_arch_vcpu_runnable(vcpu)) {
+			/*
+			 * This is the only safe place to reset unhalt flag.
+			 * otherwise it results in loosing the notification
+			 * which eventually can result in vcpu hangs.
+			 */
+			kvm_arch_vcpu_reset_pv_unhalted(vcpu);
+			/* preventing reordering should be enough here */
+			barrier();
  			kvm_make_request(KVM_REQ_UNHALT, vcpu);
  			break;
  		}


Hm, what about reusing KVM_REQ_UNHALT?


Yes, I had experimented this for some time without success.
For e.g. having
make_request(KVM_REQ_UNHALT, vcpu) directly from kick hypercall.

It would still need a flag. (did not get any alternative so far except
the  workaround posted in V4) :(

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