Re: vdso(7): new man page

[Mike Frysinger <vapier-aBrp7R+bbdUdnm+yROfE0A@public.gmane.org>]

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On Wednesday 22 May 2013 09:22:09 Michael Kerrisk wrote:
> On 04/12/13 03:28, Mike Frysinger wrote:
> > here's v2 w/Andy's feedback
> 
> Thanks for this--it's a nice piece of work. Could you take a
> look at my comments below and send a v3, please.

anything i didn't explicitly respond to below i merged with my version

> > the kernel you wish to make a syscall.
> > However, this instruction is expensive: it goes through the full
> > interrupt handling paths in the processor's microcode as well as in the
> > kernel. Newer processors have faster (but backwards incompatible)
> > instructions to initiate system calls.
> > Rather than require the C library to figure out if this functionality is
> > available at runtime itself, it can use functions provided by the kernel
> > in the vDSO.
> 
> That last point (after the comma) is the most interesting (IMO) of the use
> cases of the vDSO. If you cared to expand on the details (i.e., are what
> are mechanics of the operation of those functions provided by the kernel),
> I think that would be interesting for the reader.

i think the paragraph after this explains things somewhat as you'd like (where 
it talks about gettimeofday) ?

> > All symbols are also versioned (using the GNU version format).
> > This allows the kernel (in the very unlikely situation) to update the
> > function
> 
> s/situation/case that it is necessary/

hmm, i see what you mean, but i think your version isn't really better ... 
just different.  i'll just delete the (...) text.

> > You use the standard C calling conventions when calling any of these
> > functions. No need to worry about weird register or stack behavior.
> 
> That last sentence is a little incomplete. Could you expand/reword a little
> please.

it's meant as a follow up to the previous sentence.  so the implication is 
that there are no functions which violate the C ABI for your particular 
target.  arguments get passed in the standard way (registers/stack), and all 
the registers have corresponding behavior: scratch are scratch, caller-
preserved are caller-preserved, callee-preserved are callee-preserved, etc...

> > Note that the vDSO that is used is based on the ABI of your userspace
> > code and not the ABI of the kernel.
> > i.e. If you run an i386 32bit ELF under an i386 32bit kernel or under an
> 
> s/i.e. If/In other words, if/

i.e. shows up a lot in man pages as does e.g. (and both show up in this new 
vdso(7) page) ...

> > So when referring to sections below, use the userspace ABI.
> 
> It's not clear what you mean here when you say "use the userspace ABI."
> Could you clarify?

the two sentences that preceded this one explained things ...
-mike

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