Re: [PATCH 3/6] xentrace: P2M lookup suport for ARM platform

[Julien Grall <julien.grall@arm.com>]

On 28/03/16 19:55, Ben Sanda wrote:
> Julien and George,

Hi Ben,

Sorry for the late answer.

> Thank you for the comments. I had one question I wanted to ask.
>
>> A DOMID_XEN page could be read only too. For instance, the meta-data
>> of the trace buffer is read-only (see t_info), we don't want a domain
>> to be able to overwrite them.
>
>> However, all the foreign page are mapped read-write. You will need to
>> rework the code to map a foreign domain (see
>> XENMAPSPACE_gmfn_foreign) to allow read-only foreign page (maybe by
>> adding a new p2m_type_t?).
>
> I understand what you are saying in general, but I'm not familiar
> enough with the Xen memory mapping system to know how to actually
> implement this. The p2m_type_t p2m_ram_ro exists, which I could assign
> to read-only pages, but I'm unsure as to how to detect whether a
> request is to a read only mapping or a read-write. The normal (non
> DOMID_XEN) p2m_lookup function normally does this by reading the root-
> level page tables and somehow extracting the mapping type from the
> lpae_t structure. Given that we are not looking up the page tables for
> non-translated addresses, I'm not sure where/how to find the correct
> mapping type. Can I still lookup the page table entries for the MFN
> address and extract the p2m_type_t the same way?

You can know if the page is writable by looking to the page field 
u.inuse.type_info (PGT_writable_page means the page is writable).

Regards,

-- 
Julien Grall

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