Re: DPDK's vhost-user logging capability

["Marc-André Lureau" <mlureau@redhat.com>]

Hi

----- Original Message -----
> On Wed, Mar 23, 2016 at 03:34:09PM +0000, shesha Sreenivasamurthy (shesha)
> wrote:
> > Hi All,
> > 
> > I was going over vhost-user migration capability in DPDK in lieu of a
> > Cisco's
> > multi-q DPDK vhost-user application. I see that log_base address is
> > implemented
> > as per virtio_net device. However, desc, addr and used is per
> > vhost_virtqueue.
> > Additionally, QEMU sends one VHOST_USER_SET_LOG_BASE per queue-pair (QEMU -
> > hw/
> > virtio/vhost.c::vhost_dev_set_log).
> > 
> > Does it mean we need to log dirty pages of all rings to same location ?
> 
> Hi,
> 
> Yes, and QEMU allocates only one block of memory (see vhost_log_alloc())
> after all.

> > If that
> > is the case then why does QEMU sends separate VHOST_USER_SET_LOG_BASE per
> > queue
> > pair ?
> 
> That's kind of like a design. One queue pair is associated with one
> vhost_dev struct in QEMU, hence, all those requests will go through
> vhost_dev structs (aka, all qeueu pairs), including those that one
> time request is needed only, such as VHOST_USER_SET_MEM_TABLE. Thus,
> we introduced vhost_user_one_time_request() to avoid such case.

I am not familiar with multi-queue, but I can see that in vhost_net_start.

> So, good question, and we may need add it to the "one time request"
> group, Marc?

That would probably prevent vhost_dev_log_resize(), so it would need some kind of "force" flag. I am not sure it's worth it at this point.

> 
> And FYI, for queue-pair (or vring) request, there should be an index
> in the payload, to point to the right vring. If not, it normally
> means a global request, that _may_ need be sent once only.
> 
> 	--yliu
>