[parisc-linux] pgtable.h:acc_rights()

[Matthew Wilcox <Matthew.Wilcox@genedata.com>]

Why is acc_rights written like this?

#define acc_rights(pte)       \
        (((pte) & _PAGE_EXEC) ? \
                (((pte) & _PAGE_RW) ? \
                        acc_r(3,acc_pl(pte),acc_pl(pte)) : \
                        acc_r(3,acc_pl(pte),acc_pl(pte))) : \
                (((pte) & _PAGE_RW) ? \
                        acc_r(1,acc_pl(pte),acc_pl(pte)) : \
                        acc_r(1,acc_pl(pte),acc_pl(pte))))

According to the PA-1.1 Architecture manual I have here, if the page isn't
writable, the MSB should be clear, like so:

#define acc_rights(pte)       \
        (((pte) & _PAGE_EXEC) ? \
                (((pte) & _PAGE_RW) ? \
                        acc_r(3,acc_pl(pte),acc_pl(pte)) : \
                        acc_r(2,acc_pl(pte),acc_pl(pte))) : \
                (((pte) & _PAGE_RW) ? \
                        acc_r(1,acc_pl(pte),acc_pl(pte)) : \
                        acc_r(0,acc_pl(pte),acc_pl(pte))))

of course, it should be rewritten to look much cleaner.  In fact,
by renumbering the bits, we could get that for free (as long as we put
a big fat warning above the #defines of _PAGE_* to warn that the
constants are in that order for hardware efficiency).
              
Anyway, I'm after a spare bit in the pte for _PAGE_GATEWAY.  As I
understand it (from the mk_pte() macro), there are 12 bits available,
only 8 of which are used, so I can nobble one of them to mark a page
as being a gateway, right?

-- 
Matthew Wilcox <willy@bofh.ai>
"Windows and MacOS are products, contrived by engineers in the service of
specific companies. Unix, by contrast, is not so much a product as it is a
painstakingly compiled oral history of the hacker subculture." - N Stephenson