Re: [Qemu-devel] [RFC] vhost-user: introduce F_NEED_ALL_IOTLB protocol feature

[Jason Wang <jasowang@redhat.com>]

On 2018年04月11日 16:38, Tiwei Bie wrote:
> On Wed, Apr 11, 2018 at 04:01:19PM +0800, Jason Wang wrote:
>> On 2018年04月11日 15:20, Tiwei Bie wrote:
>>> This patch introduces VHOST_USER_PROTOCOL_F_NEED_ALL_IOTLB
>>> feature for vhost-user. By default, vhost-user backend needs
>>> to query the IOTLBs from QEMU after meeting unknown IOVAs.
>>> With this protocol feature negotiated, QEMU will provide all
>>> the IOTLBs to vhost-user backend without waiting for the
>>> queries from backend. This is helpful when using a hardware
>>> accelerator which is not able to handle unknown IOVAs at the
>>> vhost-user backend.
>>>
>>> Signed-off-by: Tiwei Bie<tiwei.bie@intel.com>
>>> ---
>>> The idea of this patch is to let QEMU push all the IOTLBs
>>> to vhost-user backend without waiting for the queries from
>>> the backend. Because hardware accelerator at the vhost-user
>>> backend may not be able to handle unknown IOVAs.
>>>
>>> This is just a RFC for now. It seems that, it doesn't work
>>> as expected when guest is using kernel driver (To handle
>>> this case, it seems that some RAM regions' events also need
>>> to be listened). Any comments would be appreciated! Thanks!
>> Interesting, a quick question is why this is needed? Can we just use exist
>> IOTLB update message?
> Yeah, we are still using the existing IOTLB update messages
> to send the IOTLB messages to backend. The only difference
> is that, QEMU won't wait for the queries before sending the
> IOTLB update messages.

Yes, my question is not very clear. I mean why must need a new feature 
bit? It looks to me qemu code can work without this.

Thanks

>
>> It looks to me at least kernel does not need this.
> Something similar in kernel vhost is that, for kernel vhost,
> QEMU needs to push the IOTLBs of some ring addrs to kernel
> vhost backend without waiting for the queries.
>
> Best regards,
> Tiwei Bie
>
>> Thanks