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 16:44 +0000, Julien Grall wrote:
> 
> 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 we should just keep this trick in our pocket for when we run out
of bits then?

Ian.


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