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

["Aravindh Puthiyaparambil (aravindp)" <aravindp@xxxxxxxxx>]

> At 19:11 +0000 on 03 Jun (1370286719), 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).
> >
> > You are right. I didn't realize magic pages are writable by the guest.
> > So this is not a good option.

BTW, are the HVM special pages (access, paging, sharing) accessible by the 
guest kernel?

> > > 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.
> >
> > I am trying to mimic what is being done in the HVM side for mem_event
> > pages. In setup_guest() (xc_hvm_build_x86.c), I see "special pages"
> > being created for console, paging, access and sharing ring pages. Then
> > xc_set_hvm_param() is used to inform the hypervisor. When a
> mem_event
> > / mem_access client comes up, it uses xc_get_hvm_param() to get the
> > pfn and maps it in. I want to do something similar for PV.
> 
> Yep.  I think it might be better to invent up a new interface for those pages,
> rather than using domain memory for them.  We can then deprecate the old
> HVM-specific params interface.

Are you saying that these pages (access, sharing, paging) should live in Dom0 
instead of the domain's memory and only be created when a mem_event listener is 
active? Or am I completely off track?

Thanks,
Aravindh


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