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[Qemu-devel] [PATCH] ppc: Fix bug in handling of PAPR hypercall exits

From: David Gibson
Subject: [Qemu-devel] [PATCH] ppc: Fix bug in handling of PAPR hypercall exits
Date: Tue, 7 Aug 2012 14:44:45 +1000

Currently for powerpc, kvm_arch_handle_exit() always returns 1, meaning
that its caller - kvm_cpu_exec() - will always exit immediately afterwards
to the loop in qemu_kvm_cpu_thread_fn().

There's no need to do this.  Once we've handled the hypercall there's no
reason we can't go straight around and KVM_RUN again, which is what ret = 0
will signal.  The only exception might be for hypercalls which affect the
state of cpu_can_run(), however the only one that might do this is H_CEDE
and for kvm that is always handled in the kernel, not qemu.

Furtherm setting ret = 0 means that when exit_requested is set from a
hypercall, we will enter KVM_RUN once more with a signal which lets the
the kernel do its internal logic to complete the hypercall with out
actually executing any more guest code.  This is important if our hypercall
also triggered a reset, which previously would re-initialize everything
without completing the hypercall.  This caused the kernel to get confused
because it thought the guest was still in the middle of a hypercall when
it has actually been reset.

This patch therefore changes to ret = 0, which is both a bugfix and a small

Signed-off-by: David Gibson <address@hidden>
 target-ppc/kvm.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 829e180..a31d278 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -766,7 +766,7 @@ int kvm_arch_handle_exit(CPUPPCState *env, struct kvm_run 
         dprintf("handle PAPR hypercall\n");
         run->papr_hcall.ret = spapr_hypercall(env, run->papr_hcall.nr,
-        ret = 1;
+        ret = 0;

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