x86/cpu/topology: Let XEN/PV use topology from CPUID/MADT

It turns out that XEN/PV Dom0 has halfways usable CPUID/MADT enumeration
except that it cannot deal with CPUs which are enumerated as disabled in
MADT.

DomU has no MADT and provides at least rudimentary topology information in
CPUID leaves 1 and 4.

For both it's important that there are not more possible Linux CPUs than
vCPUs provided by the hypervisor.

As this is ensured by counting the vCPUs before enumeration happens:

  - lift the restrictions in the CPUID evaluation and the MADT parser

  - Utilize MADT registration for Dom0

  - Keep the fake APIC ID registration for DomU

  - Fix the XEN APIC fake so the readout of the local APIC ID works for
    Dom0 via the hypercall and for DomU by returning the registered
    fake APIC IDs.

With that the XEN/PV fake approximates usefulness.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/r/20240213210252.626195405@linutronix.de

1 files changed, 8 insertions, 5 deletions

diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index 44706f01bb9b..27d1a5b7f571 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -156,11 +156,9 @@ static void __init xen_pv_smp_config(void)
 
 	topology_register_boot_apic(apicid++);
 
-	for (i = 1; i < nr_cpu_ids; i++) {
-		if (HYPERVISOR_vcpu_op(VCPUOP_is_up, i, NULL) < 0)
-			break;
+	for (i = 1; i < nr_cpu_ids; i++)
 		topology_register_apic(apicid++, CPU_ACPIID_INVALID, true);
-	}
+
 	/* Pretend to be a proper enumerated system */
 	smp_found_config = 1;
 }
@@ -451,5 +449,10 @@ void __init xen_smp_init(void)
 	/* Avoid searching for BIOS MP tables */
 	x86_init.mpparse.find_mptable		= x86_init_noop;
 	x86_init.mpparse.early_parse_smp_cfg	= x86_init_noop;
-	x86_init.mpparse.parse_smp_cfg		= xen_pv_smp_config;
+
+	/* XEN/PV Dom0 has halfways sane topology information via CPUID/MADT */
+	if (xen_initial_domain())
+		x86_init.mpparse.parse_smp_cfg	= x86_init_noop;
+	else
+		x86_init.mpparse.parse_smp_cfg	= xen_pv_smp_config;
 }