Re: [PATCH 2/6] arm64: Allow mismatched 32-bit EL0 support

[Will Deacon <will@kernel.org>]

On Mon, Nov 02, 2020 at 11:44:45AM +0000, Catalin Marinas wrote:
> On Fri, Oct 30, 2020 at 04:13:53PM +0000, Will Deacon wrote:
> > On Fri, Oct 30, 2020 at 11:18:47AM +0000, Catalin Marinas wrote:
> > > On Thu, Oct 29, 2020 at 10:20:48PM +0000, Will Deacon wrote:
> > > >     This means that if the first 32-bit-capable core is onlined late, then
> > > >     it will only get the base capabilities, but I think that's fine and
> > > >     consistent with our overall handling of hwcaps (which cannot appear
> > > >     dynamically to userspace).
> > > 
> > > Yes but such bare 32-bit mode is entirely useless and I don't think we
> > > should even pretend we have 32-bit. The compat hwcaps here would be
> > > "half thumb fastmult edsp tls idiva idivt lpae evtstrm", statically
> > > filled in. It's missing major bits like "vfp" and "neon" which are
> > > necessary for the general purpose 32-bit EABI.
> > 
> > So? If we found such a CPU during boot, would we refuse to online it because
> > we consider it "entirely useless"? No!
> 
> We _do_ online it but as a 64-bit only CPU if there were no early 32-bit
> CPUs since we are not updating the compat hwcaps anyway (and that's
> handled automatically by WEAK_LOCAL_CPU_FEATURE; we do this in a few
> places already).
> 
> > That said, given that it's _very_
> > likely for the late CPUs to support vfp and neon, we could set those caps
> > speculatively if the 64-bit cores have fpsimd (late onlining would be
> > prevented for cores lacking those). Does the architecture allow you to
> > implement both AArch64 and AArch32 at EL0, but only have fpsimd for AArch64?
> 
> Probably not but I don't want to butcher the cpufeature support further
> and have compat hwcaps derived from ID_AA64* regs. I find this hack even
> worse and I'd rather live with the partial hwcap information (and hope
> user space doesn't read hwcaps anyway ;)).
> 
> I don't see why we should change this code further when the requirement
> to the mobile vendors is to simply allow a 32-bit CPU to come up early.
> 
> > > As I said above, I think we would be even more inconsistent w.r.t.
> > > HWCAPs if we require at least one early AArch32-capable CPU, otherwise
> > > don't expose 32-bit at all. I don't see what we gain by allowing all
> > > 32-bit CPUs to come in late, other than maybe saving an entry in the
> > > cpufeature array.
> > 
> > It's a combination of there not being a good reason to prevent the
> > late-onlining and not gaining anything from the additional feature (I've
> > already shown why it doesn't help with the vast majority of callsites).
> 
> I underlined above, this is not about preventing late onlining, only
> preventing late 32-bit support. Late AArch32-capable CPUs will be
> onlined just fine, only that if we haven't got any prior 32-bit CPU, we
> no longer report the feature and the sysfs mask.

Ok. Then we're in agreement about not preventing late-onlining. The problem
then is that the existing 32-bit EL0 capability is a SYSTEM cap so even with
your diff, we still have an issue if you boot on the CPUs that support
32-bit and then try to online a 64-bit-only core (it will fail).

So I think we do need my changes to the existing cap, but perhaps we
could return false from system_supports_32bit_el0() until we've actually
seen a 32-bit capable core. That way you would keep the existing behaviour
on TX2, and we wouldn't get any unusual late-onlining failures.

I've hacked something together that seems to work, so I'll clean it up and
post it tomorrow. I've spotted a couple of pre-existing issues at the same
time, so I need to fix those first (WEAK_LOCAL_CPU_FEATURE doesn't set the
cap for late CPUs and failed onlining causes RCU stalls).

Will

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