[PATCH 4.19 109/346] x86/apic: Fix x2apic enablement without interrupt remapping

Greg Kroah-Hartman
2020-12-28T16:09:09Z

From: David Woodhouse <dwmw@amazon.co.uk>

[ Upstream commit 26573a97746c7a99f394f9d398ce91a8853b3b89 ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/include/asm/apic.h | 1 +
arch/x86/kernel/apic/apic.c | 14 ++++++++------
arch/x86/kernel/apic/x2apic_phys.c | 9 +++++++++
3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index 3c1e51ead0722..cd2aa72e21239 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -252,6 +252,7 @@ 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 e9456a2eef585..ab8187271d470 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1813,20 +1813,22 @@ 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 b5cf9e7b3830c..ed56d2850e96a 100644
--- a/arch/x86/kernel/apic/x2apic_phys.c
+++ b/arch/x86/kernel/apic/x2apic_phys.c
@@ -13,6 +13,12 @@
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)
{
@@ -103,6 +109,9 @@ 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;
}

--
2.27.0