Re: [PATCH] lib/vdso: Use __arch_use_vsyscall() to indicate fallback

[Andy Lutomirski <luto@kernel.org>]

On Sat, Oct 19, 2019 at 3:01 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Sat, 19 Oct 2019, Huacai Chen wrote:
> > On Fri, Oct 18, 2019 at 11:15 AM Andy Lutomirski <luto@kernel.org> wrote:
> > >
> > > On Thu, Oct 17, 2019 at 7:57 PM Huacai Chen <chenhc@lemote.com> wrote:
> > > >
> > > > In do_hres(), we currently use whether the return value of __arch_get_
> > > > hw_counter() is negtive to indicate fallback, but this is not a good
> > > > idea. Because:
> > > >
> > > > 1, ARM64 returns ULL_MAX but MIPS returns 0 when clock_mode is invalid;
> > > > 2, For a 64bit counter, a "negtive" value of counter is actually valid.
> > >
> > > s/negtive/negative
> > >
> > > What's the actual bug?  Is it that MIPS is returning 0 but the check
> > > is < 0?  Sounds like MIPS should get fixed.
> > My original bug is what Vincenzo said, MIPS has a boot failure if no
> > valid clock_mode, and surely MIPS need to fix. However, when I try to
> > fix it, I found that clock_getres() has another problem, because
> > __cvdso_clock_getres_common() get vd[CS_HRES_COARSE].hrtimer_res, but
> > hrtimer_res is set in update_vdso_data() which relies on
> > __arch_use_vsyscall().
>
> __arch_use_vsyscall() is a pointless exercise TBH. The VDSO data should be
> updated unconditionally so all the trivial interfaces like time() and
> getres() just work independently of the functions which depend on the
> underlying clocksource.
>
> This functions have a fallback operation already:
>
> Let __arch_get_hw_counter() return U64_MAX and the syscall fallback is
> invoked.
>

My thought was that __arch_get_hw_counter() could return last-1 to
indicate failure, which would allow the two checks to be folded into
one check.  Or we could continue to use U64_MAX and rely on the fact
that (s64)U64_MAX < 0, not worry about the cycle counter overflowing,
and letting cycles < last catch it.

(And we should change it to return s64 at some point regardless -- all
the math is signed, so the underlying types should be too IMO.)