[Qemu-devel] [PATCH 3/6] kvm: workaround a possible KVM bug when using KVM_MEM_READONLY

[Jordan Justen <jordan.l.justen@intel.com>]

On a Linux 3.8.0 based kernel, I occasionally saw a situation
where the memory region would continue to trap on memory
read even though KVM_MEM_READONLY was set.

I found that if I set the slot to a size of 0, and before
setting the slot, it would then behave as expected.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
---
 kvm-all.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kvm-all.c b/kvm-all.c
index 95e6bf2..e2ddbcb 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -205,6 +205,13 @@ static int kvm_set_user_memory_region(KVMState *s, KVMSlot *slot)
     if (s->migration_log) {
         mem.flags |= KVM_MEM_LOG_DIRTY_PAGES;
     }
+    if (mem.flags & KVM_MEM_READONLY && mem.memory_size != 0) {
+        /* Workaround an issue with setting a READONLY slot. Set the
+         * slot size to 0 before setting the slot to the desired value. */
+        mem.memory_size = 0;
+        kvm_vm_ioctl(s, KVM_SET_USER_MEMORY_REGION, &mem);
+        mem.memory_size = slot->memory_size;
+    }
     return kvm_vm_ioctl(s, KVM_SET_USER_MEMORY_REGION, &mem);
 }
 
-- 
1.7.10.4