Re: [Xen-devel] [PATCH 2 of 4 RFC] xen, pod: Check zero-check pages returned by the balloon driver

[Tim Deegan <tim@xxxxxxx>]

At 05:57 -0700 on 14 Jun (1339653458), Andres Lagar-Cavilla wrote:
> Imho, pages in any of the states of the paging state machine, are ram.
> They are not mmio, grants or whatever. They just happen to be absent at
> the moment.
> 
> This is not an immediately useful response because its corollary entails
> that PoD pages are also ram.

I'm OK with that interpretation, as long as we decide it one way or the
other and make sure that all users are correct. :)

> There are a few possible ways around this. One problem is that
> p2m_ram_paging_in may or may not have a backing mfn, so we can't just
> check the p2mt, and that is frankly annoying (plenty of wth conditionals
> in the p2m code relate to this). We could disambiguate with two states,
> e.g.:
>  p2m_ram_paging_in_absent
>  p2m_ram_paging_present
>  p2m_is_paging_in(p2mt) returns true for both -> use in most cases where
> p2m_ram_paging_in is used right now

That sounds like a good idea; for after 4.2 though. 

>  p2m_is_ram_present(p2mt) returns true for p2m_ram_rw, log dirty, shared,
> paging_out and paging_in present -> used in cases like above
>  p2m_is_ram(p2mt) includes ram_present and pod and paging_in_absent ->
> used in broader cases
> 
> That looks like a fairly dangerous changeset. But the more comprehensive
> solution. The path of least resistance is obviously to just fix George's
> current patches.

Agreed.  I was really asking how you'd like to approach this after 4.2 has
branched.

Cheers,

Tim.

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