[prev in list] [next in list] [prev in thread] [next in thread] 

List:       cap-talk
Subject:    Re: [cap-talk] [friam] Re: What is a capability?
From:       Bill Frantz <frantz () pwpconsult ! com>
Date:       2015-01-09 23:55:31
Message-ID: r422Ps-1075i-BBF2AD8AE71047AD840BF0252B551AAE () Williams-MacBook-Pro ! local
[Download RAW message or body]

On 1/8/15 at 11:42 PM, friam@googlegroups.com ('Mark S. Miller' 
via friam) wrote:

>KeyKOS is where I got the idea of membranes (from their KeySAFE design,
>although your DCCS preceded it my many years). Thus, it would be strange to
>claim that membranes are not possible in the KeyKOS family. But Kevin's
>point about pages leaves me wondering if transparent membranes are
>possible. I know you can use a Keeper to trap on attempt to access an
>unmapped page, and that the keeper can act in such a way as to emulate a
>page having been there all along. I.e., Keepers allow you to play
>fault-handling tricks like virtual memory with user code. But not by
>intercepting the memory access as a message invoked on a start key. In what
>way can a Keeper provide virtualized access to another memory page without
>providing direct access to that page?

The way we planned on making pages shared across communicating 
computers was to use a Segment Keeper to enforce the 
many-reader/single-writer model. Here's one way.

Start with the page present on all computers in R/O mode.

Some user of the page does a store into the page and that store 
traps to our segment keeper.

That segment keeper arranges with its peers to remove the page 
from all other memory maps.

It then changes the page to R/W and restarts the storing user program.

Later it marks the page R/O again and sends the changes to its 
peers who apply them and make their copies accessible R/O for 
their users.


Given the vast difference in performance between a page mapped 
into a programs address space, and one accessible through a 
mechanism such as this, we never had a use case to drive an implementation.

Cheers - Bill

-------------------------------------------------------------------------
Bill Frantz        | Airline peanut bag: "Produced  | Periwinkle
(408)356-8506      | in a facility that processes   | 16345 
Englewood Ave
www.pwpconsult.com | peanuts and other nuts." - Duh | Los Gatos, 
CA 95032


_______________________________________________
cap-talk mailing list
cap-talk@mail.eros-os.org
http://www.eros-os.org/mailman/listinfo/cap-talk
[prev in list] [next in list] [prev in thread] [next in thread]