Re: [PATCH] man2 : syscall.2 : document syscall calling conventions

[Mike Frysinger <vapier@gentoo.org>]

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On Thursday 11 April 2013 22:34:43 John David Anglin wrote:
> On 11-Apr-13, at 9:55 PM, Mike Frysinger wrote:
> > On Sunday 07 April 2013 14:48:42 John David Anglin wrote:
> >> On 7-Apr-13, at 2:39 PM, Mike Frysinger wrote:
> >>> just to be clear, the only insn you need is:
> >>> 	ble 0x100(%sr2, %r0);
> >>> 
> >>> the kernel docs say sr2 holds the kernel gateway page (so i guess
> >>> 0x100 is a
> >>> known offset into that).  the docs don't mention r0 that i can see,
> >>> so i'm
> >>> guessing it's one of those "always 0" registers ?
> >> 
> >> Yes.  There is also an entry at offset 0xb0 for light-weight-
> >> syscalls.  Currently,
> >> this implements an atomic CAS operation used for pthread support.
> > 
> > interesting.  sounds like a poor man's vDSO.  i'll document this the
> > new
> > vdso(7) man page.
> 
> Not exactly, the code runs on the gateway page which is in kernel space.
> The main reason for doing the operation in kernel space is to prevent
> processes from being preempted while executing in the lock region.  In
> general,
> parisc processes are not preempted on the gateway page.  There are
> some subtleties regarding fault handling.

sure ... the Blackfin arch does a similar thing for providing fast atomic 
primitives to userspace since the ISA can't.

what do you think of this section for vdso(7) ?  i might have to split the 
"real" vdso arches from these others since there's a couple now (arm, bfin, 
parisc), and i think there might be more down the line (microblaze).

.SS parisc (hppa) functions
.\" See linux/arch/parisc/kernel/syscall.S
.\" See linux/Documentation/parisc/registers
The parisc port has a code page full of utility functions.
Rather than use the normal ELF aux vector approach, it passes the address of
the page to the process via the SR2 register.
This is done to match the way HP-UX works.

Since it's just a raw page of code, there is no ELF information for doing
symbol lookups or versioning.
Simply call into the appropriate offset via the branch instruction, e.g.:
.br
ble <offset>(%sr2, %r0)
.if t \{\
.ft CW
\}
.TS
l l.
offset	function
_
00b0	lws_entry
00e0	set_thread_pointer
0100	linux_gateway_entry (syscall)
0268	syscall_nosys
0274	tracesys
0324	tracesys_next
0368	tracesys_exit
03a0	tracesys_sigexit
03b8	lws_start
03dc	lws_exit_nosys
03e0	lws_exit
03e4	lws_compare_and_swap64
03e8	lws_compare_and_swap
0404	cas_wouldblock
0410	cas_action
.TE
.if t \{\
.in
.ft P
\}

> There is support in glibc and libgcc for these calls.  The libgcc
> implementation
> in linux-atomic.c is very similar to that on arm.

interesting.  another arch to add :).
-mike

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