Re: [Qemu-devel] [PATCH 3/5] s390: Add new channel I/O based virtio transport.

[Stefan Hajnoczi]

On Thu, Aug 9, 2012 at 1:12 PM, Cornelia Huck <address@hidden> wrote:
> On Thu, 9 Aug 2012 12:34:04 +0100
> Stefan Hajnoczi <address@hidden> wrote:
>
>> On Tue, Aug 7, 2012 at 3:52 PM, Cornelia Huck <address@hidden> wrote:
>> > Add a new virtio transport that uses channel commands to perform
>> > virtio operations.
>> >
>> > Add a new machine type s390-ccw that uses this virtio-ccw transport
>> > and make it the default machine for s390.
>> >
>> > Signed-off-by: Cornelia Huck <address@hidden>
>> > ---
>> >  hw/qdev-monitor.c      |   5 +
>> >  hw/s390-virtio.c       | 268 ++++++++++----
>> >  hw/s390x/Makefile.objs |   1 +
>> >  hw/s390x/virtio-ccw.c  | 962 
>> > +++++++++++++++++++++++++++++++++++++++++++++++++
>> >  hw/s390x/virtio-ccw.h  |  77 ++++
>> >  vl.c                   |   1 +
>> >  6 files changed, 1243 insertions(+), 71 deletions(-)
>> >  create mode 100644 hw/s390x/virtio-ccw.c
>> >  create mode 100644 hw/s390x/virtio-ccw.h
>>
>> Is the virtqueue still using vring and assuming the hypervisor reaches
>> into guest memory?
>
> The virtqueues are guest-allocated and their location is transmitted
> via a control-type ccw to the host, which can then use it until
> notified otherwise.
>
>>
>> Can existing ccw device types access memory directly (for some reason
>> I assumed ccw always copies or send messages)?
>
> Not sure if I understand your question correctly, but read or write
> type ccws specify a memory area where the hardware/hypervisor may write
> to or read from. These accesses happen while the channel program is
> running (any time between the ssch/rsch and ending status present at
> the subchannel). The "specify an area that can be used by hardware and
> os" approach exists as well; the closed thing to the virtio-ccw
> approach is probably qdio.

Okay, thanks!

Stefan