[PATCH v2 1/3] kvm: x86: Set KVM_MAX_VCPU_ID to 4*KVM_MAX_VCPUS

[Eduardo Habkost <ehabkost@redhat.com>]

Instead of requiring KVM_MAX_VCPU_ID to be manually increased
every time we increase KVM_MAX_VCPUS, set it to 4*KVM_MAX_VCPUS.
This should be enough for CPU topologies where Cores-per-Package
and Packages-per-Socket are not powers of 2.

In practice, this increases KVM_MAX_VCPU_ID from 1023 to 1152.
The only side effect of this change is making some fields in
struct kvm_ioapic larger, increasing the struct size from 1628 to
1780 bytes (in x86_64).

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
Note: this conflicts with:
  https://lore.kernel.org/lkml/20210903130808.30142-3-jgross@suse.com
  Date: Fri,  3 Sep 2021 15:08:03 +0200
  From: Juergen Gross <jgross@suse.com>
  Subject: [PATCH v2 2/6] x86/kvm: add boot parameter for adding vcpu-id bits
  Message-Id: <20210903130808.30142-3-jgross@suse.com>

I would be happy to drop this patch and resubmit the series if
Juergen's series gets merged first.  This is part of this series
only because I'm not sure how the final version of Juergen's
changes will look like and how long they will take to be merged.
---
 arch/x86/include/asm/kvm_host.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index af6ce8d4c86a..f4cbc08b8d4d 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -39,7 +39,19 @@
 
 #define KVM_MAX_VCPUS 288
 #define KVM_SOFT_MAX_VCPUS 240
-#define KVM_MAX_VCPU_ID 1023
+
+/*
+ * In x86, the VCPU ID corresponds to the APIC ID, and APIC IDs
+ * might be larger than the actual number of VCPUs because the
+ * APIC ID encodes CPU topology information.
+ *
+ * In the worst case, we'll need less than one extra bit for the
+ * Core ID, and less than one extra bit for the Package (Die) ID,
+ * so ratio of 4 should be enough.
+ */
+#define KVM_VCPU_ID_RATIO 4
+#define KVM_MAX_VCPU_ID (KVM_MAX_VCPUS * KVM_VCPU_ID_RATIO)
+
 /* memory slots that are not exposed to userspace */
 #define KVM_PRIVATE_MEM_SLOTS 3
 
-- 
2.31.1