Re: [PATCH 12/13] x86/acpi: Do not attempt to bring up secondary CPUs in kexec case

["kirill.shutemov@linux.intel.com" <kirill.shutemov@linux.intel.com>]

On Fri, Oct 20, 2023 at 03:29:24AM +0000, Huang, Kai wrote:
> On Thu, 2023-10-05 at 16:14 +0300, Kirill A. Shutemov wrote:
> > ACPI MADT doesn't allow to offline CPU after it got woke up. It limits
> > kexec: target kernel won't be able to use more than one CPU.
> > 
> > Zero out mailbox address in the ACPI MADT wakeup structure to indicate
> > that the mailbox is not usable.
> > 
> > This is Linux-specific protocol and not reflected in ACPI spec.
> > 
> > Booting the target kernel with signle CPU is enough to cover the most
> > common case for kexec -- kdump.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >  arch/x86/kernel/acpi/madt_wakeup.c | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/acpi/madt_wakeup.c b/arch/x86/kernel/acpi/madt_wakeup.c
> > index 15bdf10b1393..4e92d1d4a5fa 100644
> > --- a/arch/x86/kernel/acpi/madt_wakeup.c
> > +++ b/arch/x86/kernel/acpi/madt_wakeup.c
> > @@ -9,6 +9,11 @@ static struct acpi_madt_multiproc_wakeup_mailbox *acpi_mp_wake_mailbox;
> >  
> >  static int acpi_wakeup_cpu(int apicid, unsigned long start_ip)
> >  {
> > +	if (!acpi_mp_wake_mailbox_paddr) {
> > +		pr_warn_once("No MADT mailbox: cannot bringup secondary CPUs. Booting with kexec?\n");
> > +		return -EOPNOTSUPP;
> > +	}
> > +
> >  	/*
> >  	 * Remap mailbox memory only for the first call to acpi_wakeup_cpu().
> >  	 *
> > @@ -78,6 +83,18 @@ int __init acpi_parse_mp_wake(union acpi_subtable_headers *header,
> >  	/* Disable CPU onlining/offlining */
> >  	cpu_hotplug_not_supported();
> >  
> > +	/*
> > +	 * ACPI MADT doesn't allow to offline CPU after it got woke up.
> > +	 * It limits kexec: target kernel won't be able to use more than
> > +	 * one CPU.
> > +	 *
> > +	 * Zero out mailbox address in the ACPI MADT wakeup structure to
> > +	 * indicate that the mailbox is not usable.
> 
> Nit:
> 
> It is better to explicitly say that this will only impact the second kernel
> because the current kernel has already detected the  mailbox address?
> 
> 	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. 

Okay. Looks good.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov