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

[Julien Grall <julien.grall@xxxxxxxxxx>]

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).

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.

-- 
Julien Grall

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