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

[Mike Frysinger <vapier@gentoo.org>]

[-- Attachment #1: Type: Text/Plain, Size: 5259 bytes --]

On Friday 12 April 2013 15:14:47 James Bottomley wrote:
> On Fri, 2013-04-12 at 14:45 -0400, Mike Frysinger wrote:
> > On Friday 12 April 2013 00:45:12 James Bottomley wrote:
> > > On Thu, 2013-04-11 at 23:38 -0400, Mike Frysinger wrote:
> > > > 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).
> > > 
> > > I've got to say, I really don't think this can be classified as a vdso.
> > > For a vdso, the kernel exports an ELF object that can be linked
> > > dynamically into any elf binary requiring it.  The ELF section
> > > information provides full details and so vdso entries can be called by
> > > symbol.
> > 
> > strictly speaking, sure, a vDSO is only a vDSO if it's an ELF (since the
> > acronym is literally "virtual dynamic shared object").  however, i see
> > the vdso as being a bit more of a flexible concept -- it's a place of
> > shared code that the kernel manages and exports for all userspace
> > processes. fundamentally, the point of the vDSO is to provide services
> > to greatly speed up userspace.  in that regard, these mapped pages are
> > exactly like vDSOs.
> 
> I don't entirely understand this classification.  If the kernel<->user
> gateway becomes classified as a vdso, that covers our syscall interface
> on every archtecture.  There's now no distinction between a vdso (which
> may not even move to kernel mode) and a syscall.
> 
> I think the difference is that a syscall is a specific call to a known
> kernel routine by number and it involves a transition to kernel mode.  A
> vdso is an exported link object containing certain functions which may
> or may not cause a trap to kernel mode when executed.  The distinction
> is how you do the call.  For syscalls, you have to know the number and
> the arguments.  For vdso you just have to know the symbol (and
> obviously, the prototype for C code) and the kernel supplies the
> implementation direct to the userspace binary.

i'm not fully versed in the parisc linux gateway page or how the architecture 
is handling things, so i could be completely off here.  from reading the source 
code, it *looked* like it was just a page of utility funcs that userspace 
branches to without changing privilege modes or going through the full syscall 
routines.

so i'm saying the gateway page itself can be thought of in the same vein as a 
vDSO.  it's a black box with entry points that provide light weight services 
to userspace.  sometimes it ends up triggering a full syscall, sometimes it 
doesn't (just like a vDSO).

> > thus i think it's appropriate to document these "fixed code" regions that
> > many arches export (ARM, Blackfin, Itanium, Microblaze, PA-RISC) in the
> > same man page as the vdso.  especially since (currently) arches do one
> > or the other, but not both.
> 
> I really see these as a type of lightweight syscall.  You use the
> syscall prototype (call by number with known arguments) but the call may
> not necessarily transition to kernel mode proper to handle the function.

if you think of the vdso in a very strict light (it's exactly an ELF that the 
kernel automatically maps into every process's address space), then i guess 
you can only classify these as lightweight syscalls (where the address/offset 
is the "syscall #").

i see vdso as being a more flexible concept than that -- if it's code mapped 
into a process's address space and provides useful lightweight services that 
are meant to be used specifically in lieu of syscall(), then it's vdso-like and 
should be in the vdso(7) man page.  it has a lot more in common imo with a 
vdso than it does with an actual syscall.  i certainly think vdso(7) is more 
appropriate for these regions than syscall(2) or syscalls(2).

> > > In the parisc gateway page implementation, we have a set of "hidden"
> > > primitives which the executable must know how to call (no self
> > > description like a vdso).  This mechanism is identical to the original
> > > intent of the x86 int <n> instruction (an instruction that traps into
> > > the kernel and performs some primitive action but to use it, you have
> > > to know which function corresponds to which value of <n>).
> > 
> > would it be useful to document all of them ?  or just the ones that
> > userspace actively uses (like syscall/cas) ?  or should all of this be
> > recorded in the kernel's Documentation/parisc/ subdir and just have the
> > man page refer people there (like it does for ARM & Blackfin currently)
> > ?
> 
> I'm not sure.  For x86 they're in include/asm/traps.h.  I think the only
> ones we really use are int3 for breakpoint, int4 for overflow and int80
> for legacy syscall.

hmm, i wasn't even considering the other arch-specific services offered by e.g. 
software interrupts.  i don't think those belong in vdso(7) as they don't 
confer any of the lightweight advantages the vdso is designed to bring, but it 
might be useful to document these somewhere.  they're also not as common for 
people to encounter as a vdso ...
-mike

[-- Attachment #2: This is a digitally signed message part. --]
[-- Type: application/pgp-signature, Size: 836 bytes --]