RE: [Xen-devel] pointers in hcalls

[Stefan Berger <stefanb@xxxxxxxxxx>]

xen-devel-bounces@xxxxxxxxxxxxxxxxxxx wrote on 02/08/2006
12:14:01 AM:

> On Tue, 2006-02-07 at 20:20 -0500, Reiner Sailer wrote:
> > 
> > The acm hypercalls uses pointers when setting and reading the
policy
> > from the hypervisor and for dumping statistics. A policy might
not
> > necessarily be less than one page. 
> > 
> > I don't remember the conversation on the Xen summit and probably
> > wasn't involved. Would you mind summarizing briefly the discussion?
> 
> (Please CC me on replies.)
> 
> The domains (especially the management tools in dom0) are passing
> virtual pointers into the hypervisor.
> 
> This assumes that the hypervisor is running in the same address space
as
> the userland tools. That assumption is not valid on PPC (we run in
real
> mode). One other port (x86-64 VT iirc) has the same problem, but since
> an x86 MMU tablewalk is pretty straightforward, they do that to
> translate the address by hand. This is not feasible on PPC.
> 
> It would dramatically simplify the problem if these memory areas were
> limited to one page. Once you go over that, the area could be machine
> discontiguous, which greatly complicates Xen's copy_to/from_user().

I wonder whether we could not implement a wrapper
for the copy_from_user function that copies the passed data on a page-by-page
basis for those arrays that stretch multiple pages? 

 Stefan

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