Messages in this thread | | | Subject | Re: [patch 6/6] Guest page hinting: s390 support. | From | Martin Schwidefsky <> | Date | Thu, 13 Mar 2008 10:36:08 +0100 |
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On Wed, 2008-03-12 at 13:45 -0700, Jeremy Fitzhardinge 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.
Vp does not happen in the current implementation. But it actually may be useful. z/VM has multiple layers of paging, the first goes to expanded storage which is very fast. If you make the page Vz and the guests needs it you have to do a standard Linux I/O to get retrieve the page. This can be slower than a read and a write to expanded storage.
> > Do the host states even really need visibility to the guest at all? It > > may be useful for the guest to be able to distinguish between Ur and Uz > > but it doesn't seem necessary. > > Well, you implicitly see the hypervisor state. If you touch a [UV]z > page then you get a fault telling you that the page has been taken away > from you (I think). And it would definitely help with debugging (seems > likely there's lots of scope for race conditions if you prematurely tell > the hypervisor you don't need the page any more...).
You get an addressing exception if you touch a Uz page. This indicates a BUG in the Linux code because this is a use after free. If the guests touches a Vz page you get a discard fault.
-- blue skies, Martin.
"Reality continues to ruin my life." - Calvin.
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