On Fri, Sep 06, 2024 at 04:26:32PM +0200, Christophe Leroy wrote: > And thanks for playing up with it while I was sleeping and getting ideas > too. > > Did you learn powerpc assembly during the night or did you know it already ?
I don't really know ppc assembly. I had perused the tree over the last week and gotten some feel for it when reviewing patches, but I don't have anything memorized (except, perhaps, the eieio instruction [1,2]). Last night after sending the first broken patch I went out to play (I play jazz guitar ~every night these days), and the whole time I kept thinking about the problem. So first thing I did when I got home was try to fake my way through some ppc asm. A fun mini project for me. [1] https://lore.kernel.org/lkml/pine.lnx.4.33.0110120919130.31677-100...@penguin.transmeta.com/ [2] https://lore.kernel.org/lkml/alpine.LFD.2.00.0904141006170.18124@localhost.localdomain/ > At the end I ended up with something which I think is simple enough for > a backport to stable. It seems like a good patch indeed, and hopefully small enough that Michael will let me carry in my tree for 6.12, per the plan. > On the long run I wonder if we should try to find a more generic > solution for getrandom instead of requiring each architecture to handle > it. On gettimeofday the selection of the right page is embeded in the > generic part, see for instance : > > static __maybe_unused __kernel_old_time_t > __cvdso_time_data(const struct vdso_data *vd, __kernel_old_time_t *time) > { > __kernel_old_time_t t; > > if (IS_ENABLED(CONFIG_TIME_NS) && > vd->clock_mode == VDSO_CLOCKMODE_TIMENS) > vd = __arch_get_timens_vdso_data(vd); > > t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec); > > if (time) > *time = t; > > return t; > } > > and powerpc just provides: > > static __always_inline > const struct vdso_data *__arch_get_timens_vdso_data(const struct > vdso_data *vd) > { > return (void *)vd + (1U << CONFIG_PAGE_SHIFT); > } It's tempting, but maybe a bit tricky. LoongArch, for example, doesn't have this problem at all, because the layout of their vvars doesn't require it. So the vd->clock_mode access is unnecessary. > Or another solution could be to put random data in a third page that is > always at the same place regardless of timens ? Maybe that's the easier way, yea. Potentially wasteful, though. Jason
