Re: [Xen-devel] Is there data copy between dom0 and domU

[xu cong <congxumail@xxxxxxxxx>]

2014-04-24 12:52 GMT-04:00 Wei Liu <wei.liu2@xxxxxxxxxx>:

On Wed, Apr 23, 2014 at 03:40:25PM -0400, xu cong wrote:
> 2014-04-23 12:22 GMT-04:00 Roger Pau Monnà <roger.pau@xxxxxxxxxx>:
>
> > On 23/04/14 18:14, Wei Liu wrote:
> > > On Wed, Apr 23, 2014 at 12:06:51PM -0400, xu cong wrote:
> > >> When applications running in domU receive/send data from/to network, is
> > >> there data copy via shared memory between dom0 and domU? Or the zero
> > copy
> > >> is used. How about the disk I/O operations? Thanks.
> > >
> > > If you're using latest kernel (3.14-ish), guest TX path is zero-copy,
> > > otherwise it's using copy. For RX path it always copies.
> > >
> > > I think disk always copies data. But I'm not sure.
> >
> > For disks, up to Linux 3.8 data is never copied and starting from 3.8
> > data can be copied (in the frontend) if both DomU and Dom0 are running
> > Linux kernel >= 3.8.
> >
> > Roger.
> >
> >
> What's the vif irq for? The following is my understanding, I am sure it is
> correct or not.
> When a packet arrives at physical machine, dom0 picks it up from NIC and
> puts it to ring buffer, then notify the destination domU. (Are all of them

Yes.

> done by vif kernel thread?) So the vif irq is issued when dom0 notifies the

For the guest RX path, yes.

> destination domU, because vif is the virtual device of domU. If it is
> correct, the vif irq (xen-dyn-event) is an event (softirq?) on the event
> channel between dom0 and domU. While, if the vif irq processing does not
> happens on the same core where destination domU is running, one IPI is
> issued to finish the notification.
>

Softirq in Linux refers to a specific type of bottom-half mechanism; IPI
in general means inter-processor interrupt. IIRC xen-dyn-event refers to
event channel event. You might want to distinguish several concepts
before moving on.

Wei.

> Regards,
> Cong

Thanks for your clarification. Is there any overhead if two ends of event channel are not on the same core compared with two ends running on the same core?

Regards,

Cong

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