[RFC PATCH v2 4/4] arm64: Export id_aar64fpr0 via sysfs

[Will Deacon]

On Wed, Oct 21, 2020 at 03:09:46PM +0100, Catalin Marinas wrote:
> On Wed, Oct 21, 2020 at 03:33:29PM +0200, Morten Rasmussen wrote:
> > On Wed, Oct 21, 2020 at 01:15:59PM +0100, Catalin Marinas wrote:
> > > one, though not as easy as automatic task placement by the scheduler (my
> > > first preference, followed by the id_* regs and the aarch32 mask, though
> > > not a strong preference for any).
> > 
> > Automatic task placement by the scheduler would mean giving up the
> > requirement that the user-space affinity mask must always be honoured.
> > Is that on the table?
> 
> I think Peter rejected it but I still find it a nicer interface from a
> dumb application perspective. It may interact badly with cpusets though
> (at least on Android).
> 
> > Killing aarch32 tasks with an empty intersection between the
> > user-space mask and aarch32_mask is not really "automatic" and would
> > require the aarch32 capability to be exposed anyway.
> 
> I agree, especially if overriding the user mask is not desirable. But if
> one doesn't play around with cpusets, 32-bit apps would run "fine" with
> the scheduler transparently placing them on the correct CPU.
> 
> Anyway, if the task placement is entirely off the table, the next thing
> is asking applications to set their own mask and kill them if they do
> the wrong thing. Here I see two possibilities for killing an app:
> 
> 1. When it ends up scheduled on a non-AArch32-capable CPU

That sounds fine to me. If we could do the exception return and take a
SIGILL, that's what we'd do, but we can't so we have to catch it before.

> 2. If the user cpumask (bar the offline CPUs) is not a subset of the
>    aarch32_mask
> 
> Option 1 is simpler but 2 would be slightly more consistent.

I disagree -- if we did this for something like fpsimd, then the consistent
behaviour would be to SIGILL on the cores without the instructions.

> There's also the question on whether the kernel should allow an ELF32 to
> be loaded (and potentially killed subsequently) if the user mask is not
> correct on execve().

I don't see the point in distinguishing between "you did execve() on a core
without 32-bit" and "you did execve() on a core with 32-bit and then
migrated to a core without 32-bit".

Will