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    SubjectRe: [patch 6/6] Guest page hinting: s390 support.
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    On Thu, 2008-03-13 at 09:17 -0700, Jeremy Fitzhardinge wrote:
    > Jeremy Fitzhardinge wrote:
    > > Martin Schwidefsky wrote:
    > >> Vz->Vr cannot happen. This would be a bug in the host.
    > >>
    > >
    > > Does that mean that Vz is effectively identical to Uz?
    >
    > Hm, on further thought:
    >
    > If guests writes to Vz pages are disallowed, then the only way out of Vz
    > is if the guest sets it to something else (Uz,Sz). If so, what's the
    > point of using that state? Why not make:
    >
    > Vr -> Uz host discard
    > Pr -> Uz host discard clean
    > Sp -> Uz set volatile
    > Uz -> Uz set volatile

    Vz is the page discarded state. The difference to Uz is slim, both
    states will cause a program check on access. Vz generates a discard
    fault, Uz generates an addressing exception which is nice for debugging.
    But I don't see a reason why an implementation that uses Uz instead of
    Vz shouldn't work.

    > But given how you've described V-state pages, I really would expect
    > writes to a Vz to work, or alternatively, all writes to V-state pages to
    > be disallowed. Are there any real uses for a writable Vr page?

    You mean in the section that speaks about the guests states S/U/V/P ?
    Always keep in mind that you can access a V/P page only until it gets
    discarded. Then the useful content of the page frame is lost and any
    read of write to the not Vz page will be answered with a discard fault.

    A Vr page is read-only. If a page gets mapped for writing it needs to
    get into the Pr state. This is the hint for the host to look at the
    dirty bit before it discards a page.
    So yes, there is no use for a writable Vr page.

    --
    blue skies,
    Martin.

    "Reality continues to ruin my life." - Calvin.




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