x86/apic: Fix x2apic enablement without interrupt remapping

Currently, Linux as a hypervisor guest will enable x2apic only if there are
no CPUs present at boot time with an APIC ID above 255.

Hotplugging a CPU later with a higher APIC ID would result in a CPU which
cannot be targeted by external interrupts.

Add a filter in x2apic_apic_id_valid() which can be used to prevent such
CPUs from coming online, and allow x2apic to be enabled even if they are
present at boot time.

Fixes: ce69a784504 ("x86/apic: Enable x2APIC without interrupt remapping under KVM")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201024213535.443185-2-dwmw2@infradead.org

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index 4e3099d9ae625f5c44e8f031f2f49f38e81ad484..57af25cb44f63253ab25e87e2f4fac8328bc486a 100644 (file)
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -259,6 +259,7 @@ static inline u64 native_x2apic_icr_read(void)
 
 extern int x2apic_mode;
 extern int x2apic_phys;
+extern void __init x2apic_set_max_apicid(u32 apicid);
 extern void __init check_x2apic(void);
 extern void x2apic_setup(void);
 static inline int x2apic_enabled(void)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index b3eef1d5c9037733e0b9a825475db203dd180953..113f6ca7b82849fd18271f3661b78b479accc2e6 100644 (file)
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1841,20 +1841,22 @@ static __init void try_to_enable_x2apic(int remap_mode)
                return;
 
        if (remap_mode != IRQ_REMAP_X2APIC_MODE) {
-               /* IR is required if there is APIC ID > 255 even when running
-                * under KVM
+               /*
+                * Using X2APIC without IR is not architecturally supported
+                * on bare metal but may be supported in guests.
                 */
-               if (max_physical_apicid > 255 ||
-                   !x86_init.hyper.x2apic_available()) {
+               if (!x86_init.hyper.x2apic_available()) {
                        pr_info("x2apic: IRQ remapping doesn't support X2APIC mode\n");
                        x2apic_disable();
                        return;
                }
 
                /*
-                * without IR all CPUs can be addressed by IOAPIC/MSI
-                * only in physical mode
+                * Without IR, all CPUs can be addressed by IOAPIC/MSI only
+                * in physical mode, and CPUs with an APIC ID that cannnot
+                * be addressed must not be brought online.
                 */
+               x2apic_set_max_apicid(255);
                x2apic_phys = 1;
        }
        x2apic_enable();

diff --git a/arch/x86/kernel/apic/x2apic_phys.c b/arch/x86/kernel/apic/x2apic_phys.c
index bc9693841353c8555c97ca01a0296727eaa61a60..e14eae6d6ea71adc7322bcb0fd8538ce0ca6dc61 100644 (file)
--- a/arch/x86/kernel/apic/x2apic_phys.c
+++ b/arch/x86/kernel/apic/x2apic_phys.c
@@ -8,6 +8,12 @@
 int x2apic_phys;
 
 static struct apic apic_x2apic_phys;
+static u32 x2apic_max_apicid __ro_after_init;
+
+void __init x2apic_set_max_apicid(u32 apicid)
+{
+       x2apic_max_apicid = apicid;
+}
 
 static int __init set_x2apic_phys_mode(char *arg)
 {
@@ -98,6 +104,9 @@ static int x2apic_phys_probe(void)
 /* Common x2apic functions, also used by x2apic_cluster */
 int x2apic_apic_id_valid(u32 apicid)
 {
+       if (x2apic_max_apicid && apicid > x2apic_max_apicid)
+               return 0;
+
        return 1;
 }