KVM: x86: Raise the hard VCPU count limit

The patch raises the hard limit of VCPU count to 254.

This will allow developers to easily work on scalability
and will allow users to test high VCPU setups easily without
patching the kernel.

To prevent possible issues with current setups, KVM_CAP_NR_VCPUS
now returns the recommended VCPU limit (which is still 64) - this
should be a safe value for everybody, while a new KVM_CAP_MAX_VCPUS
returns the hard limit which is now 254.

Cc: Avi Kivity <avi@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Pekka Enberg <penberg@kernel.org>
Suggested-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

4 files changed, 16 insertions, 4 deletions

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index b0e4b9cd6a66..75cd8fba0cde 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -175,10 +175,17 @@ Parameters: vcpu id (apic id on x86)
 Returns: vcpu fd on success, -1 on error
 
 This API adds a vcpu to a virtual machine.  The vcpu id is a small integer
-in the range [0, max_vcpus).  You can use KVM_CAP_NR_VCPUS of the
-KVM_CHECK_EXTENSION ioctl() to determine the value for max_vcpus at run-time.
+in the range [0, max_vcpus).
+
+The recommended max_vcpus value can be retrieved using the KVM_CAP_NR_VCPUS of
+the KVM_CHECK_EXTENSION ioctl() at run-time.
+The maximum possible value for max_vcpus can be retrieved using the
+KVM_CAP_MAX_VCPUS of the KVM_CHECK_EXTENSION ioctl() at run-time.
+
 If the KVM_CAP_NR_VCPUS does not exist, you should assume that max_vcpus is 4
 cpus max.
+If the KVM_CAP_MAX_VCPUS does not exist, you should assume that max_vcpus is
+same as the value returned from KVM_CAP_NR_VCPUS.
 
 On powerpc using book3s_hv mode, the vcpus are mapped onto virtual
 threads in one or more virtual CPU cores.  (This is because the
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index dd51c83aa5de..c00ec28e7147 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -26,7 +26,8 @@
 #include <asm/mtrr.h>
 #include <asm/msr-index.h>
 
-#define KVM_MAX_VCPUS 64
+#define KVM_MAX_VCPUS 254
+#define KVM_SOFT_MAX_VCPUS 64
 #define KVM_MEMORY_SLOTS 32
 /* memory slots that does not exposed to userspace */
 #define KVM_PRIVATE_MEM_SLOTS 4
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2b76ae3cb501..41dfebea6218 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2086,6 +2086,9 @@ int kvm_dev_ioctl_check_extension(long ext)
 		r = !kvm_x86_ops->cpu_has_accelerated_tpr();
 		break;
 	case KVM_CAP_NR_VCPUS:
+		r = KVM_SOFT_MAX_VCPUS;
+		break;
+	case KVM_CAP_MAX_VCPUS:
 		r = KVM_MAX_VCPUS;
 		break;
 	case KVM_CAP_NR_MEMSLOTS:
diff --git a/include/linux/kvm.h b/include/linux/kvm.h
index aace6b8691a2..206979877888 100644
--- a/include/linux/kvm.h
+++ b/include/linux/kvm.h
@@ -463,7 +463,7 @@ struct kvm_ppc_pvinfo {
 #define KVM_CAP_VAPIC 6
 #define KVM_CAP_EXT_CPUID 7
 #define KVM_CAP_CLOCKSOURCE 8
-#define KVM_CAP_NR_VCPUS 9       /* returns max vcpus per vm */
+#define KVM_CAP_NR_VCPUS 9       /* returns recommended max vcpus per vm */
 #define KVM_CAP_NR_MEMSLOTS 10   /* returns max memory slots per vm */
 #define KVM_CAP_PIT 11
 #define KVM_CAP_NOP_IO_DELAY 12
@@ -553,6 +553,7 @@ struct kvm_ppc_pvinfo {
 #define KVM_CAP_SPAPR_TCE 63
 #define KVM_CAP_PPC_SMT 64
 #define KVM_CAP_PPC_RMA	65
+#define KVM_CAP_MAX_VCPUS 66       /* returns max vcpus per vm */
 #define KVM_CAP_S390_GMAP 71
 
 #ifdef KVM_CAP_IRQ_ROUTING