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

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

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.

Thank you for pointing that out.

Cheers,
~Sergej

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