linux-kernel - [PATCH] KVM: x86: Increase KVM_MAX

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Message-Id: <20230815153537.113861-1-kyle.meyer@hpe.com>
Date:   Tue, 15 Aug 2023 10:35:39 -0500
From:   Kyle Meyer <kyle.meyer@....com>
To:     seanjc@...gle.com, pbonzini@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hasen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, vkuznets@...hat.com,
        dmatlack@...gle.com
Cc:     russ.anderson@....com, dimitri.sivanich@....com,
        steve.wahl@....com, Kyle Meyer <kyle.meyer@....com>
Subject: [PATCH] KVM: x86: Increase KVM_MAX_VCPUS to 4096

Increase KVM_MAX_VCPUS to 4096 when MAXSMP is enabled.

Notable changes (when MAXSMP is enabled):

* KMV_MAX_VCPUS will increase from 1024 to 4096.
* KVM_MAX_VCPU_IDS will increase from 4096 to 16384.
* KVM_HV_MAX_SPARSE_VCPU_SET_BITS will increase from 16 to 64.
* CPUID[HYPERV_CPUID_IMPLEMENT_LIMITS (0x40000005)].EAX will now be 4096.

* struct kvm will increase from 39408 B to 39792 B.
* struct kvm_ioapic will increase from 5240 B to 19064 B.

* The following (on-stack) bitmaps will increase from 128 B to 512 B:
	* dest_vcpu_bitmap in kvm_irq_delivery_to_apic.
	* vcpu_mask in kvm_hv_flush_tlb.
	* vcpu_bitmap in ioapic_write_indirect.
	* vp_bitmap in sparse_set_to_vcpu_mask.

Signed-off-by: Kyle Meyer <kyle.meyer@....com>
---
Virtual machines with 4096 virtual CPUs have been created on 32 socket
Cascade Lake and Sapphire Rapids systems.

4096 is the current maximum value because of the Hyper-V TLFS. See
BUILD_BUG_ON in arch/x86/kvm/hyperv.c, commit 79661c3, and Vitaly's
comment on https://lore.kernel.org/all/87r136shcc.fsf@redhat.com.

 arch/x86/include/asm/kvm_host.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 3bc146dfd38d..91a01fa17fa7 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -39,7 +39,11 @@
 
 #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
 
+#ifdef CONFIG_MAXSMP
+#define KVM_MAX_VCPUS 4096
+#else
 #define KVM_MAX_VCPUS 1024
+#endif
 
 /*
  * In x86, the VCPU ID corresponds to the APIC ID, and APIC IDs
-- 
2.35.3