On Tue, 20 Feb 2024 15:13:58 +0000 "Russell King (Oracle)" <linux@xxxxxxxxxxxxxxx> wrote: > On Tue, Feb 20, 2024 at 11:27:15AM +0000, Russell King (Oracle) wrote: > > On Thu, Feb 15, 2024 at 08:22:29PM +0100, Rafael J. Wysocki wrote: > > > On Wed, Jan 31, 2024 at 5:50 PM Russell King <rmk+kernel@xxxxxxxxxxxxxxx> wrote: > > > > diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c > > > > index cf7c1cca69dd..a68c475cdea5 100644 > > > > --- a/drivers/acpi/acpi_processor.c > > > > +++ b/drivers/acpi/acpi_processor.c > > > > @@ -314,6 +314,18 @@ static int acpi_processor_get_info(struct acpi_device *device) > > > > cpufreq_add_device("acpi-cpufreq"); > > > > } > > > > > > > > + /* > > > > + * Register CPUs that are present. get_cpu_device() is used to skip > > > > + * duplicate CPU descriptions from firmware. > > > > + */ > > > > + if (!invalid_logical_cpuid(pr->id) && cpu_present(pr->id) && > > > > + !get_cpu_device(pr->id)) { > > > > + int ret = arch_register_cpu(pr->id); > > > > + > > > > + if (ret) > > > > + return ret; > > > > + } > > > > + > > > > /* > > > > * Extra Processor objects may be enumerated on MP systems with > > > > * less than the max # of CPUs. They should be ignored _iff > > > > > > This is interesting, because right below there is the following code: > > > > > > if (invalid_logical_cpuid(pr->id) || !cpu_present(pr->id)) { > > > int ret = acpi_processor_hotadd_init(pr); > > > > > > if (ret) > > > return ret; > > > } > > > > > > and acpi_processor_hotadd_init() essentially calls arch_register_cpu() > > > with some extra things around it (more about that below). > > > > > > I do realize that acpi_processor_hotadd_init() is defined under > > > CONFIG_ACPI_HOTPLUG_CPU, so for the sake of the argument let's > > > consider an architecture where CONFIG_ACPI_HOTPLUG_CPU is set. > > > > > > So why are the two conditionals that almost contradict each other both > > > needed? It looks like the new code could be combined with > > > acpi_processor_hotadd_init() to do the right thing in all cases. > > > > > > Now, acpi_processor_hotadd_init() does some extra things that look > > > like they should be done by the new code too. > > > > > > 1. It checks invalid_phys_cpuid() which appears to be a good idea to me. > > > > > > 2. It uses locking around arch_register_cpu() which doesn't seem > > > unreasonable either. > > > > > > 3. It calls acpi_map_cpu() and I'm not sure why this is not done by > > > the new code. > > > > > > The only thing that can be dropped from it is the _STA check AFAICS, > > > because acpi_processor_add() won't even be called if the CPU is not > > > present (and not enabled after the first patch). > > > > > > So why does the code not do 1 - 3 above? > > > > Honestly, I'm out of my depth with this and can't answer your > > questions - and I really don't want to try fiddling with this code > > because it's just too icky (even in its current form in mainline) > > to be understandable to anyone who hasn't gained a detailed knowledge > > of this code. > > > > It's going to require a lot of analysis - how acpi_map_cpuid() behaves > > in all circumstances, what this means for invalid_logical_cpuid() and > > invalid_phys_cpuid(), what paths will be taken in each case. This code > > is already just too hairy for someone who isn't an experienced ACPI > > hacker to be able to follow and I don't see an obvious way to make it > > more readable. > > > > James' additions make it even more complex and less readable. > > As an illustration of the problems I'm having here, I was just writing > a reply to this with a suggestion of transforming this code ultimately > to: > > if (!get_cpu_device(pr->id)) { > int ret; > > if (!invalid_logical_cpuid(pr->id) && cpu_present(pr->id)) > ret = acpi_processor_make_enabled(pr); > else > ret = acpi_processor_make_present(pr); > > if (ret) > return ret; > } > > (acpi_processor_make_present() would be acpi_processor_hotadd_init() > and acpi_processor_make_enabled() would be arch_register_cpu() at this > point.) > > Then I realised that's a bad idea - because we really need to check > that pr->id is valid before calling get_cpu_device() on it, so this > won't work. That leaves us with: > > int ret; > > if (invalid_logical_cpuid(pr->id) || !cpu_present(pr->id)) { > /* x86 et.al. path */ > ret = acpi_processor_make_present(pr); > } else if (!get_cpu_device(pr->id)) { > /* Arm64 path */ > ret = acpi_processor_make_enabled(pr); > } else { > ret = 0; > } > > if (ret) > return ret; > > Now, the next transformation would be to move !get_cpu_device(pr->id) > into acpi_processor_make_enabled() which would eliminate one of those > if() legs. > > Now, if we want to somehow make the call to arch_regster_cpu() common > in these two paths, the next question is what are the _precise_ > semantics of acpi_map_cpu(), particularly with respect to it > modifying pr->id. Is it guaranteed to always give the same result > for the same processor described in ACPI? What acpi_map_cpu() anyway, > I can find no documentation for it. > > Then there's the question whether calling acpi_unmap_cpu() should be > done on the failure path if arch_register_cpu() fails, which is done > for the x86 path but not the Arm64 path. Should it be done for the > Arm64 path? I've no idea, but as Arm64 doesn't implement either of > these two functions, I guess they could be stubbed out and thus be > no-ops - but then we open a hole where if pr->id is invalid, we > end up passing that invalid value to arch_register_cpu() which I'm > quite sure will explode with a negative CPU number. > > So, to my mind, what you're effectively asking for is a total rewrite > of all the code in and called by acpi_processor_get_info()... and that > is not something I am willing to do (because it's too far outside of > my knowledge area.) > > As I said in my reply to patch 1, I think your comments on patch 2 > make Arm64 vcpu hotplug unachievable in a reasonable time frame, and > certainly outside the bounds of what I can do to progress this. > > So, at this point I'm going to stand down from further participation > with this patch set as I believe I've reached the limit of what I can > do to progress it. > Thanks for your hard work on this Russell - we have moved forwards. Short of anyone else stepping up I'll pick this up with the help of some my colleagues. As such I'm keen on getting patch 1 upstream ASAP so that we can exclude the need for some of the other workarounds from earlier versions of this series (the ones dropped before now). We will need a little time to get up to speed on the current status and discussion points Russell raises above. Jonathan
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