Re: [PATCH v4 12/14] arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system

[Will Deacon]

On Wed, Dec 02, 2020 at 12:59:52PM +0000, Qais Yousef wrote:
> On 12/01/20 22:13, Will Deacon wrote:
> > On Fri, Nov 27, 2020 at 01:41:22PM +0000, Qais Yousef wrote:
> > > On 11/24/20 15:50, Will Deacon wrote:
> > > > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > > > index 29017cbb6c8e..fe470683b43e 100644
> > > > --- a/arch/arm64/kernel/cpufeature.c
> > > > +++ b/arch/arm64/kernel/cpufeature.c
> > > > @@ -1237,6 +1237,8 @@ has_cpuid_feature(const struct arm64_cpu_capabilities *entry, int scope)
> > > >  
> > > >  static int enable_mismatched_32bit_el0(unsigned int cpu)
> > > >  {
> > > > +	static int lucky_winner = -1;
> > > > +
> > > >  	struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
> > > >  	bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
> > > >  
> > > > @@ -1245,6 +1247,22 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
> > > >  		static_branch_enable_cpuslocked(&arm64_mismatched_32bit_el0);
> > > >  	}
> > > >  
> > > > +	if (cpumask_test_cpu(0, cpu_32bit_el0_mask) == cpu_32bit)
> > > > +		return 0;
> > > 
> > > Hmm I'm struggling to get what you're doing here. You're treating CPU0 (the
> > > boot CPU) specially here, but I don't get why?
> > 
> > If our ability to execute 32-bit code is the same as the boot CPU then we
> > don't have to do anything. That way, we can postpone nominating the lucky
> > winner until we really need to.
> 
> Okay I see what you're doing now. The '== cpu_32bit' part of the check gave me
> trouble. If the first N cpus are 64bit only, we'll skip them here. Worth
> a comment?
> 
> Wouldn't it be better to replace this with a check if cpu_32bit_el0_mask is
> empty instead? That would be a lot easier to read.

Sorry, but I don't follow. What if all the CPUs are 32-bit capable?

I'll leave the code as-is, since I don't think it's particularly hard to
understand, and it does the right thing.

Will