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[Qemu-devel] [PATCH] target-ppc: Correct KVM synchronization for ppc_has

From: David Gibson
Subject: [Qemu-devel] [PATCH] target-ppc: Correct KVM synchronization for ppc_hash64_set_external_hpt()
Date: Fri, 1 Apr 2016 14:52:47 +1100

ppc_hash64_set_external_hpt() was added in e5c0d3c "target-ppc: Add helpers
for updating a CPU's SDR1 and external HPT".  This helper contains a
cpu_synchronize_state() since it may need to push state back to KVM

This turns out to break things when it is used in the reset path, which is
the only current user.  It appears that kvm_vcpu_dirty is not being set
early in the reset path, so the cpu_synchronize_state() is clobbering state
set up by the early part of the cpu reset path with stale state from KVM.

To fix this, remove the cpu_synchronize_state() from
ppc_hash64_set_external_hpt().  Any future non-reset-path users will need
to manually invoke cpu_synchronize_state().

Reported-by: Laurent Vivier <address@hidden>
Signed-off-by: David Gibson <address@hidden>
 target-ppc/mmu-hash64.c | 2 --
 1 file changed, 2 deletions(-)

Paolo, Peter,

This seems like the right minimal fix in the qemu-2.6 timeframe to fix
the actual bug.  However, longer term it seems like the correct thing
to do might be to set kvm_vcpu_dirty early in the reset path.  Thoughts?

diff --git a/target-ppc/mmu-hash64.c b/target-ppc/mmu-hash64.c
index 72c4ab5..caf41ce 100644
--- a/target-ppc/mmu-hash64.c
+++ b/target-ppc/mmu-hash64.c
@@ -283,8 +283,6 @@ void ppc_hash64_set_external_hpt(PowerPCCPU *cpu, void 
*hpt, int shift,
     CPUPPCState *env = &cpu->env;
     Error *local_err = NULL;
-    cpu_synchronize_state(CPU(cpu));
     if (hpt) {
         env->external_htab = hpt;
     } else {

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