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

[Michael S. Tsirkin]

On Thu, Apr 12, 2018 at 11:23:31AM +0800, Jason Wang wrote:
> 
> 
> On 2018年04月12日 01:00, Michael S. Tsirkin wrote:
> > On Wed, Apr 11, 2018 at 09:41:05PM +0800, Jason Wang wrote:
> > > 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<address@hidden>
> > > > > > ---
> > > > > > 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
> > Generally we avoid adding new messages without a protocol feature bit.
> > While careful analysis might sometimes prove it's not a strict
> > requirement, it's just overall a clean and robust approach.
> > 
> 
> Right but the looks like the patch does not introduce any new type of
> messages.
> 
> Thanks

In this case remote needs to know that it will send these messages.

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MST