Re: [Xen-devel] Proposal to allow setting up shared memory areas between VMs from xl config file

[Julien Grall <julien.grall@xxxxxxx>]

Hi Jan,

On 05/15/2017 09:52 AM, Jan Beulich wrote:
> > > > On 15.05.17 at 10:20, <julien.grall@xxxxxxx> wrote:
> > On 15/05/2017 09:08, Jan Beulich wrote:
> > > > > > On 12.05.17 at 19:01, <blackskygg@xxxxxxxxx> wrote:
> > > > ====================================================
> > > > 1. Motivation and Description
> > > > ====================================================
> > > > Virtual machines use grant table hypercalls to setup a share page for
> > > > inter-VMs communications. These hypercalls are used by all PV
> > > > protocols today. However, very simple guests, such as baremetal
> > > > applications, might not have the infrastructure to handle the grant table.
> > > > This project is about setting up several shared memory areas for inter-VMs
> > > > communications directly from the VM config file.
> > > > So that the guest kernel doesn't have to have grant table support to be
> > > > able to communicate with other guests.
> > > I think it would help to compare your proposal with the alternative of
> > > adding grant table infrastructure to such environments (which I
> > > wouldn't expect to be all that difficult). After all introduction of a
> > > (seemingly) redundant mechanism comes at the price of extra /
> > > duplicate code in the tool stack and maybe even in the hypervisor.
> > > Hence there needs to be a meaningfully higher gain than price here.
> > This is a key feature for embedded because they want to be able to share
> > buffer very easily at domain creation time between two guests.
> >
> > Adding the grant table driver in the guest OS as a high a cost when the
> > goal is to run unmodified OS in a VM. This is achievable on ARM if you
> > use passthrough.
> "high cost" is pretty abstract and vague. And I admit I have difficulty
> seeing how an entirely unmodified OS could leverage this newly
> proposed sharing model.
Let's step back for a moment, I will come back on Zhongze proposal 
afterwards.
Using grant table in the guest will obviously require the grant-table 
driver. It is not that bad. However, how do you pass the grant ref 
number to the other guest? The only way I can see is xenstore, so yet 
another driver to port.
On Zhongze proposal, the share page will be mapped at the a specific 
address in the guest memory. I agree this will require some work in the 
toolstack, on the hypervisor side we could re-use the foreign mapping 
API. But on the guest side there are nothing to do Xen specific.
What's the benefit? Baremetal guest are usually tiny, you could use the 
device-tree (and hence generic way) to present the share page for 
communicating. This means no Xen PV drivers, and therefore easier to 
move an OS in Xen VM.
Cheers,

--
Julien Grall

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