On 08/07/11 00:21, David Gilbert wrote: > On 5 July 2011 15:49, Dave Martin <dave.mar...@linaro.org> wrote: >> On Fri, Jul 1, 2011 at 6:10 PM, David Gilbert <david.gilb...@linaro.org> >> wrote: >>> Hi All, >>> I've just submitted the patches for the 64 bit atomic stuff to the >>> gcc-patches list. >>> Richard Henderson has raised the question of why the ARM commpage isn't a >>> full >>> VDSO and, if it was, then it would make the version number check a lot >>> simpler. >>> >>> What's the history behind this/how big a job is it/ >> >> I think it's there partly for historical reasons, and partly because >> there has to be a page mapped somewhere for the exception vectors >> anyway, so we may as well put other snippets of required userspace >> code there. > > OK. > >>> Dave (on holiday for a week but checking mail intermittently) >> >> Because existing userspace source and binaries already call directly >> into the helper functions in the vectors page, we can't easily >> remove/move it. >> >> We could possibly wrap the vectors page to make it look like a DSO in >> a forwards-compatible way, but since this has not happened so far it >> feels like people never saw much benefit. > > Any idea how big a job that is? > >> Can you describe the benefits with regard to this case? (And, out of >> interest, how do statically-linked programs make use of needed >> functionality in the VDSO? Browsing the web hasn't revealed any >> coherent answers to that question for me...) > > Richard's question to my patch is here: > > http://old.nabble.com/-Patch-0-3--ARM-64-bit-atomic-operations-tp31974816p31974901.html > > My patch is having to test an arbitrary address in the commpage in > init code to see if it's new enough to have the new helper that > Nicolas just added; Richard's argument is that if it was actually a > VDSO I'd just have linked against a symbol and if the symbol wasn't > there then I would have got a fairly normal linker error - I wouldn't > have needed any special code; and to be honest I think that would also > solve the problem that accessing that wacky address is also a pain for > things like qemu because we're going to have to intercept that read. > > (I've added Richard to the cc).
There is a slight performance hit to using a VDSO in that each entry will need to go through the PLT rather than jumping directly to the helper function in the kernel. R. R. _______________________________________________ linaro-dev mailing list linaro-dev@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-dev
