Re: [Qemu-devel] [PATCH 1/2] Vring: vring's listener's priority should higher than kvm

[liu ping fan]

On Thu, May 9, 2013 at 4:30 PM, Stefan Hajnoczi <address@hidden> wrote:
> On Thu, May 09, 2013 at 08:40:21AM +0800, Liu Ping Fan wrote:
>> From: Liu Ping Fan <address@hidden>
>>
>> Hosts threads which handle vring should have high MemoryListener priority
>> than kvm. For currently code, take the following scenario:
>>   kvm_region_add() run earlier before vhost_region_add(), then in guest,
>> vring's desc[i] can refer to addressX in the new region known by guest.
>> But vhost does not know this new region yet, and the vring handler will
>> fail.
>
> Is there a concrete scenario where this happens?
>
> I can think of situations like the ioeventfd being readable before
> vhost/hostmem is populated.  But I don't see how that's related to the
> priority of kvm_region_add().
>
For kvm, ie, In guest, vring_desc.addr can point to a chunk of data in
the new added memory, and kick vhost. The vhost has not added this new
region, so its local lookup table can not translate this new address,
and vring handler will fail.  If vhost priority is higher than kvm,
then, it will know this new address earlier than kvm.

> Stefan