Re: [PATCH 3/8] xen/arm: Implement p2m_type_t as an enum

[Julien Grall <julien.grall@linaro.org>]

On 12/05/2013 03:52 PM, Ian Campbell wrote:
> On Thu, 2013-12-05 at 15:42 +0000, Julien Grall wrote:
>> Until now, Xen doesn't know the type of the page (ram, foreign page, mmio,...).
>> Introduce p2m_type_t with basic types:
>>      - p2m_invalid: Nothing is mapped here
>
> Do we really need this? Is it not equivalent to not setting the present
> bit? I see x86 has the same type though -- Tim can you explain why.

We need a default value when Xen retrieves the p2m type. I don't think 
we can assume that p2m_ram_rw (or any other type) is used by default.

> Since the avail bits in the p2m pte are in pretty short supply I think
> we can avoid unnecessary types.

I plan to use directly the decimal value. So we can store up to 16 values.

>>      - p2m_ram_rw: Normal read/write guest RAM
>>      - p2m_ram_ro: Read-only guest RAM
>>      - p2m_mmio_direct: Read/write mapping of device memory
>>      - p2m_map_foreign: RAM page from foreign guest
>
> Is there no need for an entry for a grant mapping (and a ro
> counterpart)?

Hmmm .. actually grant table is mapped as RAM (so read/write and 
execute). Do we want to allow code execution from grant-mapping page?
If not, then we will need to introduce specific p2m type from grant-mapping.

-- 
Julien Grall