Re: [patch 6/6] Guest page hinting: s390 support.

[Jeremy Fitzhardinge]

Martin Schwidefsky wrote:

The state transitions (a diagram would be nicer but that is too hard
to do in ascii art...):
{Ur,Sr,Vr,Pr}: a resident page will change its block usage state if the
     guest requests it with page_set_{unused,stable,volatile}.
{Uz,Sz,Vz}: a logically zero page will change its block usage state if the
     guest requests it with page_set_{unused,stable,volatile}. The
     guest can't create the Pz state, the state will be Vz instead.
Ur -> Uz: the host system can remove an unused, resident page from memory
Sz -> Sr: on first access a stable, logically zero page will become resident
Sr -> Sp: the host system can swap a stable page to disk
Sp -> Sr: a guest access to a Sp page forces the host to retrieve it
Vr -> Vz: the host can discard a volatile page

Sp -> Vz: a page preserved by the host will be discarded if the guest sets
     the block usage state to volatile.
Pr -> Sp: the host can move a page from Pr to Sp if it discovers that the
     page is dirty while trying to discard the page. The page content is
     written to the paging device.
Pr -> Vz: the host can discard a Pr page. The Pz state is replaced by the
     Vz state.

Sp -> Uz: a page preserved by the host will be removed if the guest sets the block usage state to unused.

   J

digraph gph {
        Ur -> Sr [ label="page_set_stable" ];
        Ur -> Vr [ label="page_set_volatile" ];
        Ur -> Ur [ label="page_set_unused" ];

        Sr -> Sr [ label="page_set_stable" ];
        Sr -> Vr [ label="page_set_volatile" ];
        Sr -> Ur [ label="page_set_unused" ];

        Vr -> Sr [ label="page_set_stable" ];
        Vr -> Vr [ label="page_set_volatile" ];
        Vr -> Ur [ label="page_set_unused" ];

        Uz -> Sz [ label="page_set_stable" ];
        Uz -> Vz [ label="page_set_volatile" ];
        Uz -> Uz [ label="page_set_unused" ];

        Sz -> Sz [ label="page_set_stable" ];
        Sz -> Vz [ label="page_set_volatile" ];
        Sz -> Uz [ label="page_set_unused" ];

        Vz -> Sz [ label="page_set_stable" ];
        Vz -> Vz [ label="page_set_volatile" ];
        Vz -> Uz [ label="page_set_unused" ];

        Ur -> Uz [ label="host evict" ];

        Sz -> Sr [ label="guest write" ];
        Sr -> Sp [ label="host swap" ];
        Sp -> Sr [ label="guest access" ];

        Sp -> Uz [ label="guest discard" ];
        Sp -> Vz [ label="page_set_volatile" ];

        Pr -> Sp [ label="host discard dirty" ];
        Pr -> Vz [ label="host discard clean" ];
}

I created the attached .dot graph based purely on this description. It looks reasonable, but I didn't see how a page enters a Pr state.

gph.pdf
Description: Adobe PDF document

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