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

[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 Monné <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

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