Re: [v3 PATCH 8/8] RISC-V: Assign hwcap only according to boot cpu.

[Palmer Dabbelt]

On Mon, 11 Feb 2019 14:13:25 PST (-0800), marc.zyngier@xxxxxxx wrote:
> On Mon, 11 Feb 2019 12:03:30 -0800
> Atish Patra <atish.patra@xxxxxxx> wrote:
>
> > On 2/11/19 11:02 AM, Palmer Dabbelt wrote:
> > > On Fri, 08 Feb 2019 20:26:07 PST (-0800), david.abdurachmanov@xxxxxxxxx wrote:
> > >> On Sat, Feb 9, 2019 at 12:03 AM Atish Patra <atish.patra@xxxxxxx> wrote:
> > >>>
> > >>> On 2/8/19 1:11 AM, Christoph Hellwig wrote:
> > >>>>> +     * We don't support running Linux on hertergenous ISA systems.
> > >>>>> +     * But first "okay" processor might not be the boot cpu.
> > >>>>> +     * Check the ISA of boot cpu.
> > >>>>
> > >>>> Please use up your available 80 characters per line in comments.
> > >>>>
> > >>> I will fix it.
> > >>>
> > >>>>> +            /*
> > >>>>> +             * All "okay" hart should have same isa. We don't know how to
> > >>>>> +             * handle if they don't. Throw a warning for now.
> > >>>>> +             */
> > >>>>> +            if (elf_hwcap && temp_hwcap != elf_hwcap)
> > >>>>> +                    pr_warn("isa mismatch: 0x%lx != 0x%lx\n",
> > >>>>> +                            elf_hwcap, temp_hwcap);
> > >>>>> +
> > >>>>> +            if (hartid == boot_cpu_hartid)
> > >>>>> +                    boot_hwcap = temp_hwcap;
> > >>>>> +            elf_hwcap = temp_hwcap;
> > >>>>
> > >>>> So we always set elf_hwcap to the capabilities of the previous cpu.
> > >>>>
> > >>>>> +            temp_hwcap = 0;
> > >>>>
> > >>>> I think tmp_hwcap should be declared and initialized inside the outer loop
> > >>>> instead having to manually reset it like this.
> > >>>>
> > >>>>> +    }
> > >>>>>
> > >>>>> +    elf_hwcap = boot_hwcap;
> > >>>>
> > >>>> And then reset it here to the boot cpu.
> > >>>>
> > >>>> Shoudn't we only report the features supported by all cores?  Otherwise
> > >>>> we'll still have problems if the boot cpu supports a feature, but not
> > >>>> others.
> > >>>>
> > >>>
> > >>> Hmm. The other side of the argument is boot cpu does have a feature that
> > >>> is not supported by other hart that didn't even boot.
> > >>> The user space may execute something based on boot cpu capability but
> > >>> that won't be enabled.
> > >>>
> > >>> At least, in this way we know that we are compatible completely with
> > >>> boot cpu capabilities. Thoughts ?
> > >>
> > >> There is one example on the market, e.g., Samsung Exynos 9810.
> > >>
> > >> Mongoose 3 (big cores) only support ARMv8.0, while Cortex-A55
> > >> (little ones) support ARMv8.2 (and that brings atomics support).
> > >> I think, it's the only ARM SOC that supports different ISA extensions
> > >> between cores on the same package.
> > >>
> > >> Kernel scheduler doesn't know that big cores are missing atomics
> > >> support or that applications needs it and moves the thread
> > >> resulting in illegal instruction.
> > >>
> > >> E.g., see Golang issue: https://github.com/golang/go/issues/28431
> > >>
> > >> I also recall Jon Masters (Computer Architect at Red Hat) advocating
> > >> against having cores with mismatched capabilities on the server market.
> > >>
> > >> It just causes more problems down the line.
> > > > IMO the best bet is to only put extensions in HWCAP that are supported by all
> > > the harts that userspace will be scheduled on.
> > > Fair enough. Instead of setting HWCAP in setup_arch() once, we can set it only for boot cpu. It will be updated after every cpu comes up online.
> >
> > Thus, HWCAP will consists all extensions supported by all cpus that are online currently.
>
> You must thus prevent CPUs that have a different set of capabilities
> from coming up late (once userspace has started).

and we have no way to do that.  I'd prefer if we just looked through the entire 
device tree and only showed userspace the features that are on every possible 
CPU from the start.  Otherwise the HWCAP will shift around during a userspace 
run, which seems odd.