Re: [PATCH 1/1] x86/cpu/topology: Don't limit CPUs to 1 for Xen PV guests due to disabled APIC

[Petr Vaněk <arkamar@xxxxxxxx>]

On Sun, Apr 06, 2025 at 12:08:29PM +0200, Thomas Gleixner wrote:
> On Sat, Apr 05 2025 at 20:16, Petr Vaněk wrote:
> 
> > Xen PV guests in DomU have APIC disabled by design, which causes
> > topology_apply_cmdline_limits_early() to limit the number of possible
> > CPUs to 1, regardless of the configured number of vCPUs.
> 
> PV guests have a APIC emulation and there is no code which actually
> disables the APIC by design unconditionally. There is one way though,
> which disables the APIC indirectly.

It seems I have got a bit lost in APIC/ACPI abbreviations. Sorry.

> xen_arch_setup() disables ACPI, which in turn causes acpi_mps_check() to
> return 1, which disables the APIC. This only happens when the kernel
> configuration has:
> 
>      CONFIG_X86_MPPARSE=n
>      CONFIG_ACPI=y
> 
> If you enable MPPARSE the problem goes away, no?

Yes, it goes away.

> > +   /* 'maxcpus=0' 'nosmp' 'nolapic'
> > +    *
> > +    * The apic_is_disabled check is ignored for Xen PV domains because Xen
> > +    * disables ACPI in unprivileged PV DomU guests, which would otherwise 
> > limit
> > +    * CPUs to 1, even if multiple vCPUs were configured.
> 
> This is the wrong place as it invalidates the effect of 'nolapic' on the
> kernel command line for XEN PV.
> 
> You actually explain in the comment that XEN disables ACPI, so why are
> you slapping this xen check into this code instead of doing the obvious
> and prevent acpi_mps_check() to cause havoc?

Thank you for your explanation and suggestion. I will correct acpi_mps_check()
in following patch.

Thanks,
Petr