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    SubjectRe: [patch 6/6] Guest page hinting: s390 support.
    On Wed, 2008-03-12 at 14:36 -0700, Jeremy Fitzhardinge wrote:
    > Anthony Liguori wrote:
    > >> Vp should never happen, since you'd never preserve a V page. And
    > >> surely it would be Pr -> Sr, since the hypervisor wouldn't push the
    > >> page to backing store when you change the client state.
    > >>
    > >
    > > You're right, I meant Vp/Pp but they are invalid states. I think one of
    > > the things that keeps tripping me up is that the host can change both
    > > the host and guest page states. My initial impression was that the host
    > > handled the host state and the guest handled the guest state.
    > >
    > Yes. And it seems to me that you get unfortunate outcomes if you have a
    > Pr->Vz->Vr transition.

    Vz->Vr cannot happen. This would be a bug in the host.

    > > I was thinking that it may be useful to know a Ur verses a Uz when
    > > allocating memory. In this case, you'd rather allocate Ur pages verses
    > > Uz to avoid the fault. I don't read s390 arch code well, is the host
    > > state explicit to the guest?
    > >
    > Yes, reusing Ur pages might well be better, but who knows - they've
    > probably got an instruction which makes Uz cheap...

    Yes, faulting in a Uz page is cheap on s390. Isn't it a lovely
    architecture :-)

    > Stuff like this suggets that both parts of the state are packed
    > together, and are guest-visible:
    > + return (state & ESSA_USTATE_MASK) == ESSA_USTATE_VOLATILE &&

    Yes, the return value of the ESSA instruction has both the guest state
    and the host state.

    blue skies,

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

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