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Re: [Xen-devel] [V7 PATCH 5/7] pvh: change xsm_add_to_physmap

On Mon, 2014-02-10 at 15:16 +0000, Julien Grall wrote:
> Hi Ian,
> On 02/10/2014 01:42 PM, Ian Campbell wrote:
> > On Sun, 2014-02-09 at 16:51 +0000, Julien Grall wrote:
> >> Hello Mukesh,
> >>
> >> On 30/01/14 01:33, Mukesh Rathor wrote:
> >>>>> I'm not sure what you mean:
> >>>>>   - the code that Mukesh is adding doesn't have a struct page, it's
> >>>>>     just grabbing the foreign domid from the hypercall arg;
> >>>>>   - if we did have a struct page, we'd just need to take a ref to
> >>>>>     stop the owner changing underfoot; and
> >>>>>   - get_pg_owner() takes a domid anyway.
> >>>>
> >>>> Sorry, I was confused/mislead by the name...
> >>>>
> >>>> rcu_lock_live_remote_domain_by_id does look like what is needed.
> >>
> >> Following the xentrace tread: 
> >> http://www.gossamer-threads.com/lists/xen/devel/315883, 
> >> rcu_lock_live_remote_domain_by_id will not correctly works.
> >>
> >> On Xen on ARM, xentrace is using this hypercall to map XEN page (via 
> >> DOMID_XEN). In this case, rcu_lock_*domain* will always fails which will 
> >> prevent xentrace to works on Xen on ARM (and on PVH).
> > 
> > I'm not sure how that extends to add_to_physmap though -- doing add to
> > physmap of a DOMID_XEN owned page through the "back door" in this way
> > isn't supposed to work.
> Currently xentrace is using xc_map_foreign_page to map the trace buffer
> (with DOMID_XEN in argument).

Sorry, I misunderstood, I thought you were suggesting that rcu_lock_...
didn't work for xentrace and so couldn't work for add_to_physmap either
-- but actually xentrace ends up using add_to_physmap itself.

> AFAIK, on x86 PV domain, this called is resulting by an
> HYPERVISOR_mmu_update which allow do map xen page on priviliged domain
> (with the dummy XSM policy).
> For ARM, a call to xc_map_foreign_page will end up to
> XENMEM_add_to_physmap_range(XENMAPSPACE_gmfn_foreign).
> For both architecture, you can look at the function
> xen_remap_map_domain_mfn_range (implemented differently on ARM and x86)
> which is the last function called before going to the hypervisor.
> If we don't modify the hypercall XENMEM_add_to_physmap, we will have a
> add a new way to map Xen page for xentrace & co.

Wouldn't it be incorrect to generically return OK for mapping a
DOMID_XEN owned page -- at least something needs to validate that the
particular mfn being mapped is supposed to be shared with the guest in

TBH, it doesn't seem that this mechanism for sharing xenpages with
guests is a good fit for PVH or HVM guests/dom0. Perhaps a specific
mapspace would be more appropriate?


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