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 16:28 +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?

> > 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.

> > 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.

OK.



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