Re: monitor access to pages with a specific p2m_type_t

[sepanta s <sapanta992@gmail.com>]

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On Fri, Jun 24, 2016 at 8:10 PM, Tamas K Lengyel <tamas@tklengyel.com>
wrote:

>
> On Jun 24, 2016 05:19, "Razvan Cojocaru" <rcojocaru@bitdefender.com>
> wrote:
> >
> > On 06/24/2016 02:05 PM, George Dunlap wrote:
> > > On Wed, Jun 22, 2016 at 12:38 PM, sepanta s <sapanta992@gmail.com>
> wrote:
> > >> Hi,
> > >> Is it possible to monitor the access on the pages withp2m_type_t
> > >> p2m_ram_shared?
> > >
> > > cc'ing Tamas and Razvan
> >
> > Thanks for the CC. Judging by the "if ( npfec.write_access && (p2mt ==
> > p2m_ram_shared) )" line in hvm_hap_nested_page_fault() (from
> > xen/arch/x86/hvm/hvm.c), I'd say it certainly looks possible. But I
> > don't know what the context of the question is.
> >
> >
> > Thanks,
> > Razvan
>
The question is just getting the gfn and mfn of the page which is as type:
p2m_ram_shared to see which pages are written and unshared.

> Yes, p2m_ram_shared type pages can be monitored with mem_access just as
> normal pages. The only part that may be tricky is if you map the page into
> your monitoring application while the page is shared. Your handle will
> continue to be valid even if the page is unshared but it will continue to
> point to the shared page. However, even if you catch write access events to
> the shared page that will lead to unsharing, the mem_access notification is
> sent before unsharing. I just usually do unsharing myself in the mem_access
> callback manually for monitored pages for this reason. I might change the
> flow in 4.8 to send the notification after the unsharing happened to
> simplify this.
>
> Tamas
>
Thanks, but in mem_access , what APIs can be used to see such events ?

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