Hi James
From: James Morse [mailto:james.morse@arm.com] Sent: Friday, September 9, 2022 5:53 PM To: Salil Mehta salil.mehta@huawei.com; linaro-open-discussions@op-lists.linaro.org Cc: lorenzo.pieralisi@linaro.org; Jean-Philippe Brucker jean-philippe@linaro.org; Jonathan Cameron jonathan.cameron@huawei.com Subject: Re: [RFC PATCH v0.1 22/25] ACPI: add support to register CPUs based on the _STA enabled bit
Hi Salil,
On 09/09/2022 15:53, Salil Mehta wrote:
From: James Morse [mailto:james.morse@arm.com] Sent: Wednesday, August 31, 2022 12:09 PM To: linaro-open-discussions@op-lists.linaro.org Cc: Salil Mehta salil.mehta@huawei.com; james.morse@arm.com; lorenzo.pieralisi@linaro.org; Jean-Philippe Brucker jean-philippe@linaro.org Subject: [RFC PATCH v0.1 22/25] ACPI: add support to register CPUs based on
the
_STA enabled bit
acpi_processor_get_info() registers all present CPUs. Registering a CPU is what creates the sysfs entries and triggers the udev notifications.
arm64 virtual machines that support 'virtual cpu hotplug' use the enabled bit to indicate whether the CPU can be brought online, as the existing ACPI tables require all hardware to be described and present.
If firmware describes a CPU as present, but disabled, skip the registration. Such CPUs are present, but can't be brought online for whatever reason. (e.g. firmware/hypervisor policy).
Once firmware sets the enabled bit, the CPU can be registered and brought online by user-space. Online CPUs, or CPUs that are missing an _STA method must always be registered.
diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c index 1bd6e4b8ab66..42521d89c378 100644 --- a/drivers/acpi/acpi_processor.c +++ b/drivers/acpi/acpi_processor.c @@ -194,6 +194,32 @@ static int acpi_processor_make_present(struct acpi_processor *pr) return ret; }
+static int acpi_processor_make_enabled(struct acpi_processor *pr) +{
- unsigned long long sta;
- acpi_status status;
- bool present, enabled;
- if (!acpi_has_method(pr->handle, "_STA"))
return arch_register_cpu(pr->id);- status = acpi_evaluate_integer(pr->handle, "_STA", NULL, &sta);
- if (ACPI_FAILURE(status))
return -ENODEV;- present = sta & ACPI_STA_DEVICE_PRESENT;
- enabled = sta & ACPI_STA_DEVICE_ENABLED;
- if (cpu_online(pr->id) && (!present || !enabled)) {
pr_err_once(FW_BUG "CPU %u is online, but described as not presentor
disabled!\n", pr->id);
add_taint(TAINT_FIRMWARE_WORKAROUND, LOCKDEP_STILL_OK);- } else if (!present || !enabled) {
return -ENODEV;- }
This change and setting all possible cpus as *present* in smp_prepare_cpus() will always cause all present == possible in the guest kernel.
This is quite deliberate. I don't want to redefine present without a machine that actually supports hotplug/package-hotadd. This stuff is the tip of an ill-defined iceberg in the ACPI spec. Once there is hardware that supports this, we will have a better idea of what needs changing. Until then: everything described by ACPI must be present.
Present mask operates on the logical cpuids. Later are more closely related to the Linux abstract model. I see no problem in masking certain available devices(in this case cpus) from upper user. This is done at many places inside the kernel to intentionally not/conditionally expose certain devices to user even after getting discovered at the boot time or later.
As such, this change can co-exists irrespective of whether Hotplug or Hotadd will ever exist in the system.
I agree with the ACPI part and maybe interface is broken but then you have used ACPI_STA_DEVICE_ENABLED which has not been used yet in acpi_processor.c code which is ACPI related. How can you make sure this bit is being set by firmware of other architectures, especially legacy?
I think we can avoid that by the trick which Jean-Phillipe exploited in his patch-set[1] sent earlier last year.
That was the other side of this: https://gitlab.arm.com/linux-arm/linux-jm/-/commit/3106cccf5b9f01f44789b748 aaee3a95fee99a97
This was an attempt to do all this without changes to the ACPI spec - it doesn't touch the present cpumask.
Yes, I did refer those but the idea was not to use that change as it is.
[..]
This shall ensure that we correctly reflect only present vcpus to the linux kernel although the sizing and initialization of the GICC/GICR would have already happened for the complete set for possible vcpus i.e. the ones with
[1] _STA[0] is set & _STA[1] bit is set and [2] Either GICC_flag_Intf_Flag.Enabled set OR GICC_flag.online_capable set
so effectively we are only deferring populating the cpu present mask for the disabled cpus but which are now online capable(or Hotplug capable in future?)
What is the user observable effect of the kernel knowing this CPUs are really present?
User Interface looks inconsistent and can break existing scripts.
As you can see, user requested max possible cpus(=6) and cold booted cpus(=4) Hence, the number of cpus directories correctly being shown are 4 but then total number of cpus present are being shown as 6 (i.e. 0-5).
If we can defer the registration of the disabled cpus (but are online capable i.e. for possible - present) then I don’t see why we can't mask availability of these cpus by not marking them as present to user so that the entries are consistent. With this scripts/utils using these values can go horribly wrong.
At Guest Kernel --------------- estuary:/$ ls -al /sys/devices/system/cpu/ total 0 drwxr-xr-x 12 root 0 0 Sep 9 19:19 . drwxr-xr-x 8 root 0 0 Sep 9 19:19 .. drwxr-xr-x 7 root 0 0 Sep 9 19:19 cpu0 drwxr-xr-x 7 root 0 0 Sep 9 19:19 cpu1 drwxr-xr-x 7 root 0 0 Sep 9 19:19 cpu2 drwxr-xr-x 7 root 0 0 Sep 9 19:19 cpu3 drwxr-xr-x 2 root 0 0 Sep 9 19:19 cpufreq drwxr-xr-x 2 root 0 0 Sep 9 19:19 cpuidle [...]
estuary:/$ cat /sys/devices/system/cpu/possible 0-5 estuary:/$ cat /sys/devices/system/cpu/present 0-5 estuary:/$ cat /sys/devices/system/cpu/offline 4-5 estuary:/$
At Qemu ------- $QEMUBIN --enable-kvm -machine virt,gic-version=3 -cpu host -smp cpus=4,maxcpus=6 -append "console=ttyAMA0 root=/dev/ram earlycon rdinit=/init maxcpus=4 acpi=force"
The intention of this series is to do this as pure policy.
I anticipate pressure on the "use the MADT GICR" line, even though ACPI doesn't say anything about the presence of MADT GICC's redistributor entry. If this happens, we'd depend on present meaning present.
If we are confident that flag ACPI_STA_DEVICE_ENABLED is being set properly by ARM and other architecture firmware, then Qemu can take care of that policy. It has all the information of the vcpus which are possible and disabled (but are online capable). We can use this info to conditionally return appropriate status when _STA ACPI method is evaluated.
I intentionally refrained to use the this approach in my first RFC[1] as the default code in the acpi_processor.c was only making use of the ACPI_STA_DEVICE_PRESENT bit after evaluation of _STA method. Qemu was also setting only present bit in the returned status value. Plus, I wanted to minimize the changes in the kernel in the first version of the RFC.
[1] https://lore.kernel.org/qemu-devel/38a034f82da78b8861af6d25a83fddea@kernel.o...
All the hotplug/package-hotadd machinery is triggered by udev. We don't need to hack the cpu present mask to make that work.
May I know what exactly are your apprehensions with 'udev'?
As such 'udev' should make use of the Linux device model and it is not necessary to present 1:1 picture of the hardware to the abstract model(and which by the way we are not doing by not registering the disabled cpus). It will just expose that limited picture of the hardware to the user whatever is being presented by the kernel.
AFAICS it should work just fine but we need to limit the present cpus.
Question: Q1: Current acpi_processor.c code is not using ACPI_STA_DEVICE_{ENABLED, UI} bits. Could it break other architecture if we use these bits but some of their legacy devices or firmware does not initialize these bits to their defaults?
Almost certainly! I'm pretty confident some vendors generate their ACPI tables using markov-models. (It boots! Ship it!)
The approach that used the UI bit to mean sysfs had to be hidden behind a Kconfig symbol, which is only marginally better than #ifdef CONFIG_ARM64.
If there are problems in using the ACPI_STA_DEVICE_UI Bit because it might conflict with the legacy firmware of other architectures then let us drop that.
We can alternatively use the ACPI_STA_DEVICE_ENABLED Bit in the _STA method which can be conditionally set by the Qemu?
This new version walks a fine line described in the cover-letter: any platform with firmware tables that get this wrong should get the same user-experience as there is no policy enforcement on x86, so the !online_capable CPUs can be detected as being online, and the policy stuff gets ignored.
Yes, I do understand your predicament, but ideally user experience is dictated by what *end* user sees. Here, by not masking the disabled cpus in the cpu present mask user will not have similar experience on ARM64 and x86_64 platforms and that is undeniable and will in the end matter the most since this feature will mostly be used on the servers.
Thanks Salil