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





On 12/05/2013 04:38 PM, Ian Campbell wrote:
On Thu, 2013-12-05 at 16:28 +0000, Julien Grall wrote:

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.

"This" is what I suggested here or what you wrote already?

The code I wrote.


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.

I guess consistency is a good reason to do the same then.

Ok. So I will add p2m_grant_map_ro and p2m_grant_map_rw.

--
Julien Grall

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