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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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?

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 ;-)

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.

x86 has some special handling, but I don't know if that is for
correctness or just a historical legacy of something else.

Ian.


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