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>

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index b0e4b9cd6a663a412ab9772a220ea8447aa349dd..75cd8fba0cdef32c9d2300d83b800c6e83cc34ec 100644 (file)
--- 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 dd51c83aa5de78c99757c5dcae6fa84a4940e5ea..c00ec28e7147cf4064824ed8dca9484806ebd7f3 100644 (file)
--- 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 2b76ae3cb5012fd6afecba804bdbf92e0373fed5..41dfebea6218b7f213d35f2b32e42db6a3d8ff7b 100644 (file)
--- 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 aace6b8691a2eee8b921dc46bcd7ff3ea624e45f..206979877888b63c25e58616dec3f51cfa92a733 100644 (file)
--- 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