Re: Xen 4.7 crash

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 01/06/2016 21:45, Aaron Cornelius wrote:
>>
>>> However, since I only have 1 domain active at a time, I'm not sure why I
>> should run out of VM IDs.
>>
>> Sounds like a VMID resource leak.  Check to see whether it is freed properly
>> in domain_destroy().
>>
>> ~Andrew
> That would be my assumption.  But as far as I can tell, arch_domain_destroy() calls pwm_teardown() which calls p2m_free_vmid(), and none of the functionality related to freeing a VM ID appears to have changed in years.

The VMID handling looks suspect.  It can be called repeatedly during
domain destruction, and it will repeatedly clear the same bit out of the
vmid_mask.

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index 838d004..7adb39a 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -1393,7 +1393,10 @@ static void p2m_free_vmid(struct domain *d)
     struct p2m_domain *p2m = &d->arch.p2m;
     spin_lock(&vmid_alloc_lock);
     if ( p2m->vmid != INVALID_VMID )
-        clear_bit(p2m->vmid, vmid_mask);
+    {
+        ASSERT(test_and_clear_bit(p2m->vmid, vmid_mask));
+        p2m->vmid = INVALID_VMID;
+    }

     spin_unlock(&vmid_alloc_lock);
 }

Having said that, I can't explain why that bug would result in the
symptoms you are seeing.  It is also possibly that your issue is memory
corruption from a separate source.

Can you see about instrumenting p2m_alloc_vmid()/p2m_free_vmid() (with
vmid_alloc_lock held) to see which vmid is being allocated/freed ? 
After the initial boot of the system, you should see the same vmid being
allocated and freed for each of your domains.

~Andrew


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