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

[Tim Deegan <tim@xxxxxxx>]

B11;rgb:0000/0000/0000At 16:39 +0000 on 05 Dec (1386257979), Ian Campbell wrote:
> 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?

Sure.

Tim.

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