On Mon, May 04, 2020 at 08:23:08PM +0530, Anshuman Khandual wrote: > > > On 05/04/2020 06:13 PM, Mark Rutland wrote: > > On Mon, May 04, 2020 at 06:00:00PM +0530, Anshuman Khandual wrote: > >> A global boot_cpu_data is not really required. Lets drop this. > > > > I don't think it's true that this isn't required today. > > > > One reason that we have both boot_cpu_data and a cpu_data variable for > > CPU0 is that CPU0 itself can be hotplugged out then back in, and this > > allows us to detect if CPU0's features have changed (e.g. due to FW > > failing to configure it appropriately, or real physical hotplug > > occurring). > > Understood. After hotplug, CPU0 will come back via secondary_start_kernel() > where it's current register values will be checked against earlier captured > values i.e boot_cpu_data. > > But wondering why should CPU0 be treated like any other secondary CPU. IOW > in case the fresh boot CPU register values dont match with boot_cpu_data, > should not the online process just be declined ? AFAICS, current approach > will let the kernel run with taint in case of a mismatch.
I don't follow. When CPU0 is hotplguged back in it'll follow the secondary boot path, so it can be rejected as with any other secondary CPU. If I'm missing a case, could you please point that out more specifically? > > So NAK to the patch as it stands. If we're certain we capture all of > > those details even without boot_cpu_data, then we should make other > > changes to make that clear (e.g. removing it as an argument to > > update_cpu_features()). > > There might not be another way, unless we can override CPU0's cpu_data > variable when the boot CPU comes back in after vetting against existing > values. Is there any particular reason to store the very first boot CPU0 > info for ever ? The reason is so that we can log the values for comparison. Otherwise we'll have to choose some arbitrary CPU's value in order to do so. > Passing on CPU0's cpu_data variable in update_cpu_features() for secondary > CPUs during boot still make sense. It helps in finalizing register values. > Re-entering CPU0's test against boot_cpu_data seems different. I think that practically this means we should leave this as-is. If we need to keep it around for CPU, then we may as well keep it around and use it consitently for all secondary CPUs. I'd prefer to leave this as-is given it's simple to reason about. Thanks, Mark.
