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 17:31 +0100, Tim Deegan wrote:
> At 16:19 +0000 on 05 Dec (1386256758), Ian Campbell wrote:
> > On Thu, 2013-12-05 at 17:07 +0100, Tim Deegan wrote:
> > > At 15:52 +0000 on 05 Dec (1386255150), 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.
> > > 
> > > There are other not-present types on x86: PoD, paged-out, emulated-MMIO.  
> > 
> > This means not Present doesn't imply invalid, I see. Even if we don't
> > currently have such types I can see how this makes sense. In theory when
> > we get short of bits we could consider the P bit one of the type bits,
> > which would give us 16 present and 16 not-present types. Overkill for
> > now I expect.
> > 
> > Is it ever the case that an actual p2m entry contains the p2m_invalid
> > bit pattern
> 
> Yes, IIRC if an existing entry is removed, it's replaced with an
> explicit invalid entry.  It used to be the case that inclid was type 0
> so by default all uninitialised entries were invalid too, but ram_rw
> had to be type 0 (becasue of an inconvenient interacion with AMD
> IOMMUs).  For added confusion on x86 we still have the legacy rule
> that invalid entries are reported as mmio_dm, because the default path
> is to ask qemu.

I think we avoid all those pitfalls on arm right now, so probably
invalid == P==0b0,AVAIL=0b0000 makes sense?

Ian.


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