Re: [Xen-devel] [PATCH v2 09/10] xen/arm: Set foreign page type to p2m_map_foreign

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-12-10 at 01:51 +0000, Julien Grall wrote:
> 
> On 12/09/2013 04:40 PM, Ian Campbell wrote:
> > On Mon, 2013-12-09 at 03:34 +0000, Julien Grall wrote:
> >> Xen needs to know that the current page belongs to another domain. Also 
> >> take
> >> the refcount to this page.
> >>
> >> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> >>
> >> ---
> >>      Changes in v2:
> >>          - Even if gcc is buggy (see 
> >> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18501)
> >>          define p2m type per mapspace to let the compiler warns about 
> >> unitialized
> >>          values.
> >> ---
> >>   xen/arch/arm/mm.c |   24 ++++++++++++++++--------
> >>   1 file changed, 16 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> >> index ba51f6e..b08ffa0 100644
> >> --- a/xen/arch/arm/mm.c
> >> +++ b/xen/arch/arm/mm.c
> >> @@ -993,6 +993,7 @@ static int xenmem_add_to_physmap_one(
> >>   {
> >>       unsigned long mfn = 0;
> >>       int rc;
> >> +    p2m_type_t t;
> >>
> >>       switch ( space )
> >>       {
> >> @@ -1025,18 +1026,22 @@ static int xenmem_add_to_physmap_one(
> >>
> >>           d->arch.grant_table_gpfn[idx] = gpfn;
> >>
> >> +        t = p2m_ram_rw;
> >> +
> >>           spin_unlock(&d->grant_table->lock);
> >>           break;
> >>       case XENMAPSPACE_shared_info:
> >> -        if ( idx == 0 )
> >> -            mfn = virt_to_mfn(d->shared_info);
> >> -        else
> >> +        if ( idx != 0 )
> >>               return -EINVAL;
> >> +
> >> +        mfn = virt_to_mfn(d->shared_info);
> >> +        t = p2m_ram_rw;
> >> +
> >>           break;
> >>       case XENMAPSPACE_gmfn_foreign:
> >>       {
> >> -        paddr_t maddr;
> >>           struct domain *od;
> >> +        struct page_info *page;
> >>           od = rcu_lock_domain_by_any_id(foreign_domid);
> >>           if ( od == NULL )
> >>               return -ESRCH;
> >> @@ -1048,15 +1053,18 @@ static int xenmem_add_to_physmap_one(
> >>               return rc;
> >>           }
> >>
> >> -        maddr = p2m_lookup(od, pfn_to_paddr(idx));
> >> -        if ( maddr == INVALID_PADDR )
> >> +        /* Take refcount to the foreign domain page.
> >
> > "refcount on...". Or "Take a reference to..."
> >
> >> +         * Refcount will be release in XENMEM_remove_from_physmap */
> >
> > and a few other places...
> >
> > Like with the unmap it might be best to push this reference manipulation
> > down into the eventual function which makes the mapping. That would keep
> > this stuff much more localised.
> 
> Forget to answer to this part:
> 
> We need to take the reference with foreign domain in parameter, 
> otherwise we don't really know if the foreign domain has unlikely 
> removed the mapping between the hypercall was called and now.

Are we not holding the p2m lock at this point?

We probably do need to do the final check and unmap with the lock held.
Either by moving the unmap within the same lock as the lookup or perhaps
with some sort of cmpxchg mechanism where we pass in the pte value we
think we are tearing down. Just checking the type may not be sufficient
if the guest tears down one foreign mapping and replaces it with
another.

Ian.


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