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From: David Woodhouse <dwmw2@infradead.org>
To: x86@kernel.org
Cc: iommu <iommu@lists.linux-foundation.org>,
	kvm <kvm@vger.kernel.org>,
	linux-hyperv@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>
Subject: [PATCH 01/13] x86/apic: Use x2apic in guest kernels even with unusable CPUs.
Date: Mon,  5 Oct 2020 16:28:44 +0100
Message-ID: <20201005152856.974112-1-dwmw2@infradead.org> (raw)
In-Reply-To: <77e64f977f559412f62b467fd062d051ea288f14.camel@infradead.org>

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

If the BIOS has enabled x2apic mode, then leave it enabled and don't
fall back to xapic just because some CPUs can't be targeted.

Previously, if there are CPUs with APIC IDs > 255, the kernel will
disable x2apic and fall back to xapic. And then not use the additional
CPUs anyway, since xapic can't target them either.

In fact, xapic mode can target even *fewer* CPUs, since it can't use
the one with APIC ID 255 either. Using x2apic instead gives us at least
one more CPU without needing interrupt remapping in the guest.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/include/asm/apic.h        |  1 +
 arch/x86/kernel/apic/apic.c        | 18 ++++++++++++++----
 arch/x86/kernel/apic/x2apic_phys.c |  9 +++++++++
 3 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index 1c129abb7f09..b0fd204e0023 100644
--- 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 b3eef1d5c903..a75767052a92 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1841,14 +1841,24 @@ 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
+		/*
+		 * If there are APIC IDs above 255, they cannot be used
+		 * without interrupt remapping. In a virtual machine, but
+		 * not on bare metal, X2APIC can be used anyway. In the
+		 * case where BIOS has enabled X2APIC, don't disable it
+		 * just because there are APIC IDs that can't be used.
+		 * They couldn't be used if the kernel falls back to XAPIC
+		 * anyway.
 		 */
 		if (max_physical_apicid > 255 ||
 		    !x86_init.hyper.x2apic_available()) {
 			pr_info("x2apic: IRQ remapping doesn't support X2APIC mode\n");
-			x2apic_disable();
-			return;
+			if (!x2apic_mode) {
+				x2apic_disable();
+				return;
+			}
+
+			x2apic_set_max_apicid(255);
 		}
 
 		/*
diff --git a/arch/x86/kernel/apic/x2apic_phys.c b/arch/x86/kernel/apic/x2apic_phys.c
index bc9693841353..b4b4e89c1118 100644
--- 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;
+
+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;
 }
 
-- 
2.26.2


  reply index

Thread overview: 51+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-05 15:28 [PATCH 0/13] Fix per-domain IRQ affinity, allow >255 CPUs on x86 without IRQ remapping David Woodhouse
2020-10-05 15:28 ` David Woodhouse [this message]
2020-10-05 15:28   ` [PATCH 02/13] x86/msi: Only use high bits of MSI address for DMAR unit David Woodhouse
2020-10-06 20:45     ` Thomas Gleixner
2020-10-05 15:28   ` [PATCH 03/13] x86/ioapic: Handle Extended Destination ID field in RTE David Woodhouse
2020-10-05 15:28   ` [PATCH 04/13] x86/apic: Support 15 bits of APIC ID in IOAPIC/MSI where available David Woodhouse
2020-10-05 15:28   ` [PATCH 05/13] genirq: Prepare for default affinity to be passed to __irq_alloc_descs() David Woodhouse
2020-10-06 21:01     ` Thomas Gleixner
2020-10-06 21:07       ` David Woodhouse
2020-10-05 15:28   ` [PATCH 06/13] genirq: Add default_affinity argument " David Woodhouse
2020-10-06 21:06     ` Thomas Gleixner
2020-10-05 15:28   ` [PATCH 07/13] irqdomain: Add max_affinity argument to irq_domain_alloc_descs() David Woodhouse
2020-10-06 21:26     ` Thomas Gleixner
2020-10-07  7:19       ` David Woodhouse
2020-10-07 13:37         ` Thomas Gleixner
2020-10-07 14:10           ` David Woodhouse
2020-10-07 15:57             ` Thomas Gleixner
2020-10-07 16:11               ` David Woodhouse
2020-10-07 20:53                 ` Thomas Gleixner
2020-10-08  7:21               ` David Woodhouse
2020-10-08  9:34                 ` Thomas Gleixner
2020-10-08 11:10                   ` David Woodhouse
2020-10-08 12:40                     ` Thomas Gleixner
2020-10-09  7:54                       ` David Woodhouse
2020-10-05 15:28   ` [PATCH 08/13] genirq: Add irq_domain_set_affinity() David Woodhouse
2020-10-06 21:32     ` Thomas Gleixner
2020-10-07  7:22       ` David Woodhouse
2020-10-05 15:28   ` [PATCH 09/13] x86/irq: Add x86_non_ir_cpumask David Woodhouse
2020-10-06 21:42     ` Thomas Gleixner
2020-10-07  7:25       ` David Woodhouse
2020-10-05 15:28   ` [PATCH 10/13] x86/irq: Limit IOAPIC and MSI domains' affinity without IR David Woodhouse
2020-10-06 21:54     ` Thomas Gleixner
2020-10-07  7:48       ` David Woodhouse
2020-10-07 12:59         ` Thomas Gleixner
2020-10-07 13:08           ` David Woodhouse
2020-10-07 14:05             ` Thomas Gleixner
2020-10-07 14:23               ` David Woodhouse
2020-10-07 16:02                 ` Thomas Gleixner
2020-10-07 16:15                   ` David Woodhouse
2020-10-07 15:05               ` David Woodhouse
2020-10-07 15:25                 ` Thomas Gleixner
2020-10-07 15:46                   ` David Woodhouse
2020-10-07 17:23                     ` Thomas Gleixner
2020-10-07 17:34                       ` David Woodhouse
2020-10-05 15:28   ` [PATCH 11/13] x86/smp: Allow more than 255 CPUs even without interrupt remapping David Woodhouse
2020-10-05 15:28   ` [PATCH 12/13] iommu/irq_remapping: Kill most of hyperv-iommu.c now it's redundant David Woodhouse
2020-10-05 15:28   ` [PATCH 13/13] x86/kvm: Add KVM_FEATURE_MSI_EXT_DEST_ID David Woodhouse
2020-10-07  8:14     ` Paolo Bonzini
2020-10-07  8:59       ` David Woodhouse
2020-10-07 11:15         ` Paolo Bonzini
2020-10-07 12:04           ` David Woodhouse

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