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

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

On 12/05/2013 04:14 PM, Ian Campbell wrote:
> On Thu, 2013-12-05 at 16:01 +0000, Julien Grall wrote:
>>
>> 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.
>
> 16 is short supply in my book ;-)
>
> Having got a bit further through the series I see how p2m_invalid is
> being used now. It is a useful pseudo-type but it doesn't need to be
> represented in the avail bits I don't think. How about:
>
> typedef enum {
>      p2m_ram_rw,         /* Normal read/write guest RAM */
>      p2m_ram_ro,         /* Read-only; writes are silently dropped */
>      p2m_mmio_direct,    /* Read/write mapping of genuine MMIO area /
>      p2m_map_foreign,    /* Ram pages from foreign domain */
>
>      p2m_max_real_type = 16,    /* Types after this are pseudo-types. */
>
>      p2m_invalid,        /* Nothing mapped here */
>
> } p2m_type_t;
>
> BUILD_BUG_ON(p2m_max_real_type >= 2^4);
>
> Now you can return it etc but it never needs to get put in an actual
> pte?


This solution was easier to avoid extra code in the different function.
I will rework it for the next series.

>
> Maybe this is one for the future when we get a bit short on bits.
>
>>>>       - 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.
>
> If a guest is stupid enough to execute code from a page owned by another
> guest then it gets what it deserves ;-)

Actually X86, disable execution on grant and foreign mapping.

> My question wasn't about that though -- just whether it is useful for
> Xen to track whether the particular RAM mapping is normal or a grant
> mapping.

For now, I don't see a specific reason to track it.

-- 
Julien Grall