| Message ID | 20250405181650.22827-2-arkamar@atlas.cz (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | x86/topology: Fix regression limiting Xen PV DomU CPUs to 1 | expand |
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. 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? > + /* '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? Thanks, tglx
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
diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c index 01456236a6dd..10aa7f471ec9 100644 --- a/arch/x86/kernel/cpu/topology.c +++ b/arch/x86/kernel/cpu/topology.c @@ -428,8 +428,13 @@ void __init topology_apply_cmdline_limits_early(void) { unsigned int possible = nr_cpu_ids; - /* 'maxcpus=0' 'nosmp' 'nolapic' */ - if (!setup_max_cpus || apic_is_disabled) + /* '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. + */ + if (!setup_max_cpus || (!xen_pv_domain() && apic_is_disabled)) possible = 1; /* 'possible_cpus=N' */
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. This is a regression introduced in version 6.9 in commit 7c0edad3643f ("x86/cpu/topology: Rework possible CPU management") which added an early check that limits CPUs if apic_is_disabled, without accounting for the fact that Xen PV guests always disable APIC even when SMP is supported. This patch fixes the issue by skipping the apic_is_disabled check for Xen PV guests, allowing them to boot with the full set of configured vCPUs. Fixes: 7c0edad3643f ("x86/cpu/topology: Rework possible CPU management") Cc: Thomas Gleixner <tglx@linutronix.de> Cc: x86@kernel.org Cc: xen-devel@lists.xenproject.org Cc: stable@vger.kernel.org # 6.9+ Signed-off-by: Petr Vaněk <arkamar@atlas.cz> --- arch/x86/kernel/cpu/topology.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)