Re: [PATCH v5 00/15] An alternative series for asymmetric AArch32 systems

[Will Deacon <will@kernel.org>]

Hi Peter,

Cheers for taking a look.

On Tue, Dec 15, 2020 at 06:36:45PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 08, 2020 at 01:28:20PM +0000, Will Deacon wrote:
> > The aim of this series is to allow 32-bit ARM applications to run on
> > arm64 SoCs where not all of the CPUs support the 32-bit instruction set.
> > Unfortunately, such SoCs are real and will continue to be productised
> > over the next few years at least. I can assure you that I'm not just
> > doing this for fun.
> > 
> > Changes in v5 include:
> > 
> >   * Teach cpuset_cpus_allowed() about task_cpu_possible_mask() so that
> >     we can avoid returning incompatible CPUs for a given task. This
> >     means that sched_setaffinity() can be used with larger masks (like
> >     the online mask) from userspace and also allows us to take into
> >     account the cpuset hierarchy when forcefully overriding the affinity
> >     for a task on execve().
> > 
> >   * Honour task_cpu_possible_mask() when attaching a task to a cpuset,
> >     so that the resulting affinity mask does not contain any incompatible
> >     CPUs (since it would be rejected by set_cpus_allowed_ptr() otherwise).
> > 
> >   * Moved overriding of the affinity mask into the scheduler core rather
> >     than munge affinity masks directly in the architecture backend.
> 
> Hurmph... so if I can still read, this thing will auto truncate the
> affinity mask to something that only contains compatible CPUs, right?
> 
> Assuming our system has 8 CPUs (0xFF), half of which are 32bit capable
> (0x0F), then, when our native task (with affinity 0x3c) does a
> fork()+execve() of a 32bit thingy the resulting task has 0x0c.
> 
> If that in turn does fork()+execve() of a native task, it will retain
> the trucated affinity mask (0x0c), instead of returning to the wider
> mask (0x3c).
> 
> IOW, any (accidental or otherwise) trip through a 32bit helper, will
> destroy user state (the affinity mask: 0x3c).

Yes, that's correct, and I agree that it's a rough edge. If you're happy
with the idea of adding an extra mask to make this work, then I can start
hacking that up (although I doubt I'll get something out before the new
year at this point).

> Should we perhaps split task_struct::cpus_mask, one to keep an original
> copy of the user state, and one to be an effective cpumask for the task?
> That way, the moment a task constricts or widens it's
> task_cpu_possible_mask() we can re-compute the effective mask without
> loss of information.

Hmm, we might already have most of the pieces in place for this (modulo
the extra field), since cpuset_cpus_allowed() provides the limiting mask
now so this might be relatively straightforward.

Famous last words...

Will