On Wed, Nov 16, 2016 at 05:02:59PM -0500, Christopher Covington wrote:
> On 11/16/2016 12:46 PM, Marc Zyngier wrote:
> > On 16/11/16 14:38, Andrew Jones wrote:
> >> ARMv7-A isn't exactly the same as ARMv8-A32 (AArch32). This
> >> function allows unit tests to make the distinction.
> >
> > Hi Drew,
> >
> > Overall, having to find out about the architecture is a bad idea most of
> > the time. We have feature registers for most things, and it definitely
> > makes more sense to check for those than trying to cast a wider net.
> >
> >>
> >> Signed-off-by: Andrew Jones <drjones@xxxxxxxxxx>
> >>
> >> ---
> >> I'm actually unsure if there's a feature bit or not that I could
> >> probe instead. It'd be nice if somebody can confirm. Thanks, drew
>
> I'd be happy to settle with the hard-coded CPU list.
>
> But if you're curious about alternatives, I've taken a look through some
> documentation. ID_ISAR0.coproc describes whether mrrc is available but
> I think it is generally available on v7 and above. I think ID_ISAR5 will
> be zero on v7 and nonzero on v8-A32. But PMCR.LC seems like the best bit
> to check.
>
> >> diff --git a/lib/arm64/asm/processor.h b/lib/arm64/asm/processor.h
> >> index 84d5c7ce752b..b602e1fbbc2d 100644
> >> --- a/lib/arm64/asm/processor.h
> >> +++ b/lib/arm64/asm/processor.h
> >> @@ -81,5 +81,10 @@ DEFINE_GET_SYSREG32(mpidr)
> >> extern void start_usr(void (*func)(void *arg), void *arg, unsigned long sp_usr);
> >> extern bool is_user(void);
> >>
> >> +static inline bool is_aarch32(void)
> >> +{
> >> + return false;
> >> +}
> >> +
> >> #endif /* !__ASSEMBLY__ */
> >> #endif /* _ASMARM64_PROCESSOR_H_ */
> >>
> >
> > So the real question is: what are you trying to check for?
>
> The question is "how many bits wide is pmccntr?" I think we
> can test whether writing PMCR.LC = 1 sticks. Based on the
> documentation, it seems to me like it wouldn't for v7 and
> would for v8-A32.
>
> uint8_t size_pmccntr(void) {
> uint32_t pmcr = get_pmcr();
> if (pmcr & PMU_PMCR_LC_MASK)
> return 64;
> set_pmcr(pmcr | (1 << PMU_PMCR_LC_SHIFT));
> if (get_pmcr() & PMU_PMCR_LC_MASK)
> return 64;
> return 32;
> }
Thanks for this Cov. I mostly agree with the proposal, except for two
reasons; the spec says the bit is UNK/SBZP for v7, which doesn't give
me 100% confidence that we can rely on it behaving a certain way. And,
I'd rather not rely on the emulation we're testing to work well enough
that we can probe it.
For our PMU testing purposes there is a somewhat ugly, but foolproof
way to manage it; we could just add a command line input that says
we're aarch32 (-cpu host,aarch64=off -append aarch32)
I'm still hoping we can come up with a general way to implement an
is_aarch32() though, assuming we'll want it elsewhere in tests.
Although that assumption may be wrong...
Thanks,
drew
>
> Thanks,
> Cov
>
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