Messages in this thread | ![/](/images/icornerl.gif) | | From | Thomas Gleixner <> | Subject | Re: [PATCHv4 13/14] x86/acpi: Do not attempt to bring up secondary CPUs in kexec case | Date | Fri, 15 Dec 2023 21:08:06 +0100 |
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On Tue, Dec 05 2023 at 03:45, Kirill A. Shutemov wrote: > ACPI MADT doesn't allow to offline CPU after it got woke up. It limits
to offline a CPU after it was onlined. This limits kexec: ...
> kexec: the second kernel won't be able to use more than one CPU.
... one CPU, which is enough to cover the kdump case.
> Now acpi_mp_wake_mailbox_paddr already has the mailbox address. > The acpi_wakeup_cpu() will use it to bring up secondary cpus. > > Zero out mailbox address in the ACPI MADT wakeup structure to indicate > that the mailbox is not usable. This prevents the kexec()-ed kernel > from reading a vaild mailbox, which in turn makes the kexec()-ed kernel > only be able to use the boot CPU. > > This is Linux-specific protocol and not reflected in ACPI spec. > > Booting the second kernel with signle CPU is enough to cover the most > common case for kexec -- kdump.
This is confusing at best and I doubt that kdump is the most common case for every one.
To prevent a kexec kernel from onlining secondary CPUs invalidate the mailbox address in the ACPI MADT wakeup structure which prevents a kexec kernel to use it.
This is safe as the booting kernel has the mailbox address cached already and acpi_wakeup_cpu() uses the cached value to bring up the secondary CPUs.
Note: This is a Linux specific convention and not covered by the ACPI specification.
Hmm?
> + /* > + * ACPI MADT doesn't allow to offline CPU after it got woke up.
to offline a CPU after it was onlined.
> + * It limits kexec: the second kernel won't be able to use more than
This limits kexec: ...
> + * one CPU. > + * > + * Now acpi_mp_wake_mailbox_paddr already has the mailbox address. > + * The acpi_wakeup_cpu() will use it to bring up secondary cpus. > + * > + * Zero out mailbox address in the ACPI MADT wakeup structure to > + * indicate that the mailbox is not usable. This prevents the > + * kexec()-ed kernel from reading a vaild mailbox, which in turn > + * makes the kexec()-ed kernel only be able to use the boot CPU. > + * > + * This is Linux-specific protocol and not reflected in ACPI spec.
See changelog comment...
> + */ > + mp_wake->mailbox_address = 0; > + > apic_update_callback(wakeup_secondary_cpu_64, acpi_wakeup_cpu); > > return 0;
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