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