[PATCH] kvm/powerpc: Handle errors in secondary thread grabbing

In the Book3s HV code, kvmppc_run_core() has logic to grab the secondary
threads of the physical core.

If for some reason a thread is stuck, kvmppc_grab_hwthread() can fail,
but currently we ignore the failure and continue into the guest. If the
stuck thread is in the kernel badness ensues.

Instead we should check for failure and bail out.

I've moved the grabbing prior to the startup of runnable threads, to simplify
the error case. AFAICS this is harmless, but I could be missing something
subtle.

Signed-off-by: Michael Ellerman <redacted>
---

Or we could just BUG_ON() ?
---
 arch/powerpc/kvm/book3s_hv.c |   22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 721d460..55925cd 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c

@@ -884,16 +884,30 @@ static int kvmppc_run_core(struct kvmppc_vcore *vc)
 		if (vcpu->arch.ceded)
 			vcpu->arch.ptid = ptid++;
 
+	/*
+	 * Grab any remaining hw threads so they can't go into the kernel.
+	 * Do this early to simplify the cleanup path if it fails.
+	 */
+	for (i = ptid; i < threads_per_core; ++i) {
+		int j, rc = kvmppc_grab_hwthread(vc->pcpu + i);
+		if (rc) {
+			for (j = i - 1; j ; j--)
+				kvmppc_release_hwthread(vc->pcpu + j);
+
+			list_for_each_entry(vcpu, &vc->runnable_threads,
+					    arch.run_list)
+				vcpu->arch.ret = -EBUSY;
+
+			goto out;
+		}
+	}
+
 	vc->stolen_tb += mftb() - vc->preempt_tb;
 	vc->pcpu = smp_processor_id();
 	list_for_each_entry(vcpu, &vc->runnable_threads, arch.run_list) {
 		kvmppc_start_thread(vcpu);
 		kvmppc_create_dtl_entry(vcpu, vc);
 	}
-	/* Grab any remaining hw threads so they can't go into the kernel */
-	for (i = ptid; i < threads_per_core; ++i)
-		kvmppc_grab_hwthread(vc->pcpu + i);
-
 	preempt_disable();
 	spin_unlock(&vc->lock);
 

-- 
1.7.9.5

Michael,

On Tue, Oct 16, 2012 at 11:15:50AM +1100, Michael Ellerman wrote:

In the Book3s HV code, kvmppc_run_core() has logic to grab the secondary
threads of the physical core.

If for some reason a thread is stuck, kvmppc_grab_hwthread() can fail,
but currently we ignore the failure and continue into the guest. If the
stuck thread is in the kernel badness ensues.

Instead we should check for failure and bail out.

I've moved the grabbing prior to the startup of runnable threads, to simplify
the error case. AFAICS this is harmless, but I could be missing something
subtle.

Thanks for looking at this - but in fact this is fixed by my patch
entitled "KVM: PPC: Book3S HV: Fix some races in starting secondary
threads" submitted back on August 28.

Regards,
Paul.

On Tue, 2012-10-16 at 14:13 +1100, Paul Mackerras wrote:

Michael,

On Tue, Oct 16, 2012 at 11:15:50AM +1100, Michael Ellerman wrote:

quoted

In the Book3s HV code, kvmppc_run_core() has logic to grab the secondary
threads of the physical core.

If for some reason a thread is stuck, kvmppc_grab_hwthread() can fail,
but currently we ignore the failure and continue into the guest. If the
stuck thread is in the kernel badness ensues.

Instead we should check for failure and bail out.

I've moved the grabbing prior to the startup of runnable threads, to simplify
the error case. AFAICS this is harmless, but I could be missing something
subtle.

Thanks for looking at this - but in fact this is fixed by my patch
entitled "KVM: PPC: Book3S HV: Fix some races in starting secondary
threads" submitted back on August 28.

OK thanks. It seems that patch didn't make 3.7 ?

I don't see it in kvm-ppc-next either.

cheers

On Tue, 2012-10-16 at 17:00 +1100, Michael Ellerman wrote:

quoted

Thanks for looking at this - but in fact this is fixed by my patch
entitled "KVM: PPC: Book3S HV: Fix some races in starting secondary
threads" submitted back on August 28.

OK thanks. It seems that patch didn't make 3.7 ?

I don't see it in kvm-ppc-next either.

Alex, WTF ?

Ben.

On 16.10.2012, at 21:33, Benjamin Herrenschmidt wrote:

On Tue, 2012-10-16 at 17:00 +1100, Michael Ellerman wrote:

quoted

quoted

Thanks for looking at this - but in fact this is fixed by my patch
entitled "KVM: PPC: Book3S HV: Fix some races in starting secondary
threads" submitted back on August 28.

=20
OK thanks. It seems that patch didn't make 3.7 ?
=20
I don't see it in kvm-ppc-next either.

=20
Alex, WTF ?

Hrm. Not sure what happened there. I think I waited for your ack, but =
never actually applied things when it came. My bad :)


Alex