Re: [Xen-devel] Creating a magic page for PV mem_access

[Tim Deegan <tim@xxxxxxx>]

Hi,

At 01:24 +0000 on 01 Jun (1370049844), Aravindh Puthiyaparambil (aravindp) 
wrote:
> I am trying to create a magic / special page for the PV mem_access. I
> am mimicking what is being done for the console page
> (alloc_magic_pages()) on the tools side. On the hypervisor side, I am
> planning on stashing the address of this page in the pv_domain
> structure akin to how the special pages are stored in params[] of the
> hvm_domain structure.

OK, can you back up a bit and describe what you're going to use this
page for?  A PV domain's 'magic' pages may not be quite what you want.
First, they're owned by the guest, so the guest can write to them (and
so they can't be trusted for doing hypervisor->dom0 communications).
And second, I'm not sure that mem_access pages really need to
saved/restored with the rest of the VM -- I'd have thought that you
could just set up a new, empty ring on the far side.

> With HVM domains, xc_set_hvm_param() is used by
> the tools to populate this on the hypervisor side. What is the method
> I should follow to do this for PV domains? I see how things work for
> the console, xenconsole pages as they get passed through the
> start_info structure. Do I need to implement an equivalent
> xc_set_pv_param() or do I use the start_info page to store the
> mem_access magic page address?

Again, that depends on what the page is used for.  If the guest needs to
access it, then it needs to find out about it somehow, but the usual way
to pass that kind of config info to the guest is using Xenstore.  If the
guest needs to be involved right form the start of day (i.e. before
Xenbus gets going) it might have to go in the start_info, but that's a
less attractive option.

Cheers,

Tim.

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