kvm: introduce KVM_MAX_VCPU_ID

The KVM_MAX_VCPUS define provides the maximum number of vCPUs per guest, and
also the upper limit for vCPU ids. This is okay for all archs except PowerPC
which can have higher ids, depending on the cpu/core/thread topology. In the
worst case (single threaded guest, host with 8 threads per core), it limits
the maximum number of vCPUS to KVM_MAX_VCPUS / 8.

This patch separates the vCPU numbering from the total number of vCPUs, with
the introduction of KVM_MAX_VCPU_ID, as the maximal valid value for vCPU ids
plus one.

The corresponding KVM_CAP_MAX_VCPU_ID allows userspace to validate vCPU ids
before passing them to KVM_CREATE_VCPU.

This patch only implements KVM_MAX_VCPU_ID with a specific value for PowerPC.
Other archs continue to return KVM_MAX_VCPUS instead.

Suggested-by: Radim Krcmar <rkrcmar@redhat.com>
Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index 8cc857fffcc774d18fd56bf1491adf4afc7aa52a..a4482cce4bae043e6e734841f28294e5150d9870 100644 (file)
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -199,8 +199,8 @@ Type: vm ioctl
 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).
+This API adds a vcpu to a virtual machine. No more than max_vcpus may be added.
+The vcpu id is an integer in the range [0, max_vcpu_id).
 
 The recommended max_vcpus value can be retrieved using the KVM_CAP_NR_VCPUS of
 the KVM_CHECK_EXTENSION ioctl() at run-time.
@@ -212,6 +212,12 @@ 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.
 
+The maximum possible value for max_vcpu_id can be retrieved using the
+KVM_CAP_MAX_VCPU_ID of the KVM_CHECK_EXTENSION ioctl() at run-time.
+
+If the KVM_CAP_MAX_VCPU_ID does not exist, you should assume that max_vcpu_id
+is the same as the value returned from KVM_CAP_MAX_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
 hardware requires all the hardware threads in a CPU core to be in the

diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
index d7b343170453df82b4f31429020c8680e1f949bf..a07645c17818e50dacad5af2fe36c309a3462666 100644 (file)
--- a/arch/powerpc/include/asm/kvm_host.h
+++ b/arch/powerpc/include/asm/kvm_host.h
@@ -40,6 +40,9 @@
 #define KVM_MAX_VCORES         NR_CPUS
 #define KVM_USER_MEM_SLOTS     512
 
+#include <asm/cputhreads.h>
+#define KVM_MAX_VCPU_ID                (threads_per_subcore * KVM_MAX_VCORES)
+
 #ifdef CONFIG_KVM_MMIO
 #define KVM_COALESCED_MMIO_PAGE_OFFSET 1
 #endif

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 0a0e00d9c5dadba0f3fd28d076ac5413812f7203..352889d6e322c44304f50bb016f7a5d71c220aea 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -35,6 +35,10 @@
 
 #include <asm/kvm_host.h>
 
+#ifndef KVM_MAX_VCPU_ID
+#define KVM_MAX_VCPU_ID KVM_MAX_VCPUS
+#endif
+
 /*
  * The bit 16 ~ bit 31 of kvm_memory_region::flags are internally used
  * in kvm, other bits are visible for userspace which are defined in

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index a7f1f8032ec1fbcb27bc452d818a71269af81a2e..05ebf475104ca121646d99eab8a5e1738404ef6f 100644 (file)
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -865,6 +865,7 @@ struct kvm_ppc_smmu_info {
 #define KVM_CAP_SPAPR_TCE_64 125
 #define KVM_CAP_ARM_PMU_V3 126
 #define KVM_CAP_VCPU_ATTRIBUTES 127
+#define KVM_CAP_MAX_VCPU_ID 128
 
 #ifdef KVM_CAP_IRQ_ROUTING
 

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4fd482fb9260b89feda9736854861719b50c974f..ed3d9bb18a5666b325d94a185d03fe76511cf45e 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2272,7 +2272,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
        int r;
        struct kvm_vcpu *vcpu;
 
-       if (id >= KVM_MAX_VCPUS)
+       if (id >= KVM_MAX_VCPU_ID)
                return -EINVAL;
 
        vcpu = kvm_arch_vcpu_create(kvm, id);
@@ -2746,6 +2746,8 @@ static long kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
        case KVM_CAP_MULTI_ADDRESS_SPACE:
                return KVM_ADDRESS_SPACE_NUM;
 #endif
+       case KVM_CAP_MAX_VCPU_ID:
+               return KVM_MAX_VCPU_ID;
        default:
                break;
        }