Re: [musl] Re: New powerpc vdso calling convention

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Rich Felker's message of April 26, 2020 2:22 am:
> On Sat, Apr 25, 2020 at 08:56:54PM +1000, Nicholas Piggin wrote:
>> >> The ELF v2 ABI convention would suit it well, because the caller already
>> >> requires the function address for ctr, so having it in r12 will
>> >> eliminate the need for address calculation, which suits the vdso data
>> >> page access.
>> >> 
>> >> Is there a need for ELF v1 specific calls as well, or could those just be
>> >> deprecated and remain on existing functions or required to use the ELF
>> >> v2 calls using asm wrappers?
>> > 
>> > What's ELF v1 and ELF v2 ? Is ELF v1 what PPC32 uses ? If so, I'd say 
>> > yes, it would be good to have it to avoid going through ASM in the middle..
>> 
>> I'm not sure about PPC32. On PPC64, ELFv2 functions must be called with 
>> their address in r12 if called at their global entry point. ELFv1 have a 
>> function descriptor with call address and TOC in it, caller has to load 
>> the TOC if it's global.
>> 
>> The vdso doesn't have TOC, it has one global address (the vdso data 
>> page) which it loads by calculating its own address.
> 
> A function descriptor could be put in the VDSO data page, or as it's
> done now by glibc the vdso linkage code could create it. My leaning is
> to at least have a version of the code that's callable (with the right
> descriptor around it) by v1 binaries, but since musl does not use
> ELFv1 at all we really have no stake in this and I'm fine with
> whatever outcome users of v1 decide on.

I agree, I think it would be good to make it look as much like a normal
function as possible.

>> The kernel doesn't change the vdso based on whether it's called by a v1 
>> or v2 userspace (it doesn't really know itself and would have to export 
>> different functions). glibc has a hack to create something:
> 
> I'm pretty sure it does know because signal invocation has to know
> whether the function pointer points to a descriptor or code. At least
> for FDPIC archs (similar to PPC64 ELFv1 function descriptors) it knows
> and has to know.

It knows on a per-executable basis (by looking at the ELF header). It 
doesn't know per-system though so we can't patch the vdso accordingly. 
But we could include both sets of entry points and map in the 
appropriate one at exec time I think.

>> >> Is there a good reason for the system call fallback to go in the vdso
>> >> function rather than have the caller handle it?
>> > 
>> > I've seen at least one while porting powerpc to the C VDSO: arguments 
>> > toward VDSO functions are in volatile registers. If the caller has to 
>> > call the fallback by itself, it has to save them before calling the 
>> > VDSO, allthought in 99% of cases it won't use them again. With the 
>> > fallback called by the VDSO itself, the arguments are still hot in 
>> > volatile registers and ready for calling the fallback. That make it very 
>> > easy to call them, see patch 5 in the series 
>> > (https://patchwork.ozlabs.org/project/linuxppc-dev/patch/59bea35725ab4cefc67a678577da8b3ab7771af5.1587401492.git.christophe.leroy@c-s.fr/)
> 
> This is actually a good reason not to spuriously fail and fallback. At
> present musl wouldn't take advantage of it because musl uses the
> fallback path for lazy initialization of the vdso function pointer and
> doesn't special-case the MIPS badness, but if it made a big difference
> we probably could shuffle things around to only do the fallback on
> archs that need it and avoid saving the input arg registers across the
> vdso call.

It's a point for it yes. I don't know if any libc or app would want to 
instrument it or do special accounting or something for system calls.

Thanks,
Nick