[hppa-linux] syscall work
Thu, 25 Mar 1999 17:40:11 -0600
>> I'd suggest *not* using the same gateway page address as HP-UX. If you
>> do, you won't be able to develop a later kernel extension to support
>> HP-UX binaries, unless you allocate syscall numbers carefully.
>Doesn't that depend which gateway page you map into each application ?
Here's one of the subtleties of PA-RISC VM. Under HP-UX, there's only one
mapping (space 0, offset 0xC0000000) that is shared by all applications.
The libc stub branches to (%sr7,0xC0000000), and %sr7 always contains
a zero while running user apps.
Sharing a mapping between applications is a performance win, because
you can share the TLB entry and you can share the physical page without
worrying about any cache flushing issues. Since PA-RISC has virtually-
indexed caches, you can only share physical pages under limited conditions
-- see appendix F in the 2.0 arch book.
Of course, you don't *have* to share those mappings, but you will
have more TLB misses and (unless you're careful about virtual address
allocation for aliasing) more cache flushing.