[PATCH v4 04/14] arm64: Kill 32-bit applications scheduled on 64-bit-only CPUs

[Will Deacon]

On Wed, Dec 02, 2020 at 01:52:16PM +0000, Qais Yousef wrote:
> On 12/01/20 16:56, Will Deacon wrote:
> > On Fri, Nov 27, 2020 at 01:12:17PM +0000, Qais Yousef wrote:
> > > On 11/24/20 15:50, Will Deacon wrote:
> > > > Scheduling a 32-bit application on a 64-bit-only CPU is a bad idea.
> > > > 
> > > > Ensure that 32-bit applications always take the slow-path when returning
> > > > to userspace on a system with mismatched support at EL0, so that we can
> > > > avoid trying to run on a 64-bit-only CPU and force a SIGKILL instead.
> > > > 
> > > > Signed-off-by: Will Deacon <will at kernel.org>
> > > > ---
> > > 
> > > nit: We drop this patch at the end. Can't we avoid it altogether instead?
> > 
> > I did it like this so that the last patch can be reverted for
> > testing/debugging, but also because I think it helps the structure of the
> > series.
> 
> Cool. I had a comment about the barrier(), you were worried about
> cpu_affinity_invalid() being inlined by the compiler and then things get
> mangled such that TIF_NOTIFY_RESUME clearing is moved after the call as you
> described? Can the compiler move things if cpu_affinity_invalid() is a proper
> function call (not inlined)?

I think function calls implicitly clobber memory, but you'd have to annotate
the thing as noinline to prevent it being inlined.

Will