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

[xu cong <congxumail@xxxxxxxxx>]

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

Regards,

Cong

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