x86/vsmp: Fix number of CPUs when vsmp is disabled

In case CONFIG_X86_VSMP is not set, limit the number of CPUs to
the number of CPUs of the first board.

Also make CONFIG_X86_VSMP depend on CONFIG_SMP, as there's
little point in having a vsmp machine with a single CPU.
Signed-off-by: NShai Fultheim <shai@scalemp.com>
[ido@wizery.com: rebased, fixed minor coding-style issues]
Signed-off-by: NIdo Yariv <ido@wizery.com>
Signed-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/Kconfig

@@ -376,6 +376,7 @@ config X86_VSMP
 	select PARAVIRT
 	depends on X86_64 && PCI
 	depends on X86_EXTENDED_PLATFORM
+	depends on SMP
 	---help---
 	  Support for ScaleMP vSMP systems.  Say 'Y' here if this kernel is
 	  supposed to run on these EM64T-based machines.  Only choose this option

arch/x86/kernel/vsmp_64.c

@@ -15,6 +15,7 @@
 #include <linux/init.h>
 #include <linux/pci_ids.h>
 #include <linux/pci_regs.h>
+#include <linux/smp.h>
 
 #include <asm/apic.h>
 #include <asm/pci-direct.h>
@@ -22,6 +23,8 @@
 #include <asm/paravirt.h>
 #include <asm/setup.h>
 
+#define TOPOLOGY_REGISTER_OFFSET 0x10
+
 #if defined CONFIG_PCI && defined CONFIG_PARAVIRT
 /*
  * Interrupt control on vSMPowered systems:
@@ -149,12 +152,49 @@ int is_vsmp_box(void)
 	return 0;
 }
 #endif
+
+static void __init vsmp_cap_cpus(void)
+{
+#if !defined(CONFIG_X86_VSMP) && defined(CONFIG_SMP)
+	void __iomem *address;
+	unsigned int cfg, topology, node_shift, maxcpus;
+
+	/*
+	 * CONFIG_X86_VSMP is not configured, so limit the number CPUs to the
+	 * ones present in the first board, unless explicitly overridden by
+	 * setup_max_cpus
+	 */
+	if (setup_max_cpus != NR_CPUS)
+		return;
+
+	/* Read the vSMP Foundation topology register */
+	cfg = read_pci_config(0, 0x1f, 0, PCI_BASE_ADDRESS_0);
+	address = early_ioremap(cfg + TOPOLOGY_REGISTER_OFFSET, 4);
+	if (WARN_ON(!address))
+		return;
+
+	topology = readl(address);
+	node_shift = (topology >> 16) & 0x7;
+	if (!node_shift)
+		/* The value 0 should be decoded as 8 */
+		node_shift = 8;
+	maxcpus = (topology & ((1 << node_shift) - 1)) + 1;
+
+	pr_info("vSMP CTL: Capping CPUs to %d (CONFIG_X86_VSMP is unset)\n",
+		maxcpus);
+	setup_max_cpus = maxcpus;
+	early_iounmap(address, 4);
+#endif
+}
+
 void __init vsmp_init(void)
 {
 	detect_vsmp_box();
 	if (!is_vsmp_box())
 		return;
 
+	vsmp_cap_cpus();
+
 	set_vsmp_pv_ops();
 	return;
 }