Re: [Xen-devel] [PATCH 14/18] arm/altp2m: Add HVMOP_altp2m_set_mem_access.

[Julien Grall <julien.grall@xxxxxxx>]

On 05/07/16 22:55, Sergej Proskurin wrote:
> Hello Julien,
>
> On 07/05/2016 02:49 PM, Julien Grall wrote:
> > Hello Sergej,
> >
> > On 04/07/16 12:45, Sergej Proskurin wrote:
> > > +static inline
> > > +int p2m_set_altp2m_mem_access(struct domain *d, struct p2m_domain *hp2m,
> > > +                              struct p2m_domain *ap2m, p2m_access_t a,
> > > +                              gfn_t gfn)
> > > +{
> > [...]
> >
> > > +    /* Set mem access attributes - currently supporting only one (4K)
> > > page. */
> > > +    mask = level_masks[3];
> > > +    return apply_p2m_changes(d, ap2m, INSERT,
> > > +                             gpa & mask,
> > > +                             (gpa + level_sizes[level]) & mask,
> > > +                             maddr & mask, mattr, 0, p2mt, a);
> > The operation INSERT will remove the previous mapping and decrease page
> > reference for foreign mapping (see p2m_put_l3_page). So if you set the
> > memory access for this kind of page, the reference count will be wrong
> > afterward.
> I see your point. As far as I understand, the purpose of the function
> p2m_put_l3_page to simply decrement the ref count of the particular pte
> and free the page if its' ref count reaches zero. Since p2m_put_l3_page
> is called only twice in p2m.c, we could insert a check
> (p2m_is_hostp2m/altp2m) making sure that the ref count is decremented
> only if the p2m in question is the hostp2m. This will make sure that the
> ref count is maintained correctly.
Why don't you use the operation MEMACCESS?

Regards,

--
Julien Grall

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