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

[Tiwei Bie <tiwei.bie@intel.com>]

On Thu, Apr 12, 2018 at 03:38:50PM +0800, Jason Wang wrote:
> On 2018年04月12日 09:44, Tiwei Bie wrote:
> > On Wed, Apr 11, 2018 at 08:37:17PM +0300, Michael S. Tsirkin wrote:
> > > On Wed, Apr 11, 2018 at 04:38:53PM +0800, 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.
> > > So I have a concern with that, in that without any flow
> > > control the socket buffer used by vhost-user might become
> > > full.
> > Each IOTLB update message needs a reply. So I think it
> > won't happen.
> 
> Is this what we've already done now? I don't find any statement on this in
> vhost-user.txt?
> 
> """
>  * VHOST_USER_IOTLB_MSG
> 
>       Id: 22
>       Equivalent ioctl: N/A (equivalent to VHOST_IOTLB_MSG message type)
>       Master payload: struct vhost_iotlb_msg
>       Slave payload: u64
> 
>       Send IOTLB messages with struct vhost_iotlb_msg as payload.
>       Master sends such requests to update and invalidate entries in the
> device
>       IOTLB. The slave has to acknowledge the request with sending zero as
> u64
>       payload for success, non-zero otherwise.
>       This request should be send only when VIRTIO_F_IOMMU_PLATFORM feature
>       has been successfully negotiated.
> """

Yeah, it's what we've already done now. It's in the above
statement you quoted:

"The slave has to acknowledge the request with sending zero as
u64 payload for success, non-zero otherwise."

> 
> And you probably need to modify the following statement:
> 
> """
> The master isn't expected to take the initiative to send IOTLB update
> messages,
> as the slave sends IOTLB miss messages for the guest virtual memory areas it
> needs to access.
> """

Yeah, you're right. Thanks! This statement needs some minor updates.

Best regards,
Tiwei Bie

> 
> Thanks
> 
> 
> > 
> > Best regards,
> > Tiwei Bie
> > 
>