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KVM: x86: Allow userspace to set maximum VCPU id for VM 

Introduce new max_vcpu_ids in KVM for x86 architecture. Userspace
can assign maximum possible vcpu id for current VM session using
KVM_CAP_MAX_VCPU_ID of KVM_ENABLE_CAP ioctl().

This is done for x86 only because the sole use case is to guide
memory allocation for PID-pointer table, a structure needed to
enable VMX IPI.

By default, max_vcpu_ids set as KVM_MAX_VCPU_IDS.

Suggested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Zeng Guang <guang.zeng@intel.com>
Message-Id: <20220419154444.11888-1-guang.zeng@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Zeng Guang 2022-04-19 23:44:44 +08:00 committed by Paolo Bonzini
parent 1d5e740d51
commit 3587531638
3 changed files with 47 additions and 0 deletions
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21
Documentation/virt/kvm/api.rst
View File

@ -7494,6 +7494,27 @@ The valid bits in cap.args[0] are:
generate a #UD within the guest.
=================================== ============================================
7.32 KVM_CAP_MAX_VCPU_ID
------------------------
:Architectures: x86
:Target: VM
:Parameters: args[0] - maximum APIC ID value set for current VM
:Returns: 0 on success, -EINVAL if args[0] is beyond KVM_MAX_VCPU_IDS
supported in KVM or if it has been set.
This capability allows userspace to specify maximum possible APIC ID
assigned for current VM session prior to the creation of vCPUs, saving
memory for data structures indexed by the APIC ID. Userspace is able
to calculate the limit to APIC ID values from designated
CPU topology.
The value can be changed only until KVM_ENABLE_CAP is set to a nonzero
value or until a vCPU is created. Upon creation of the first vCPU,
if the value was set to zero or KVM_ENABLE_CAP was not invoked, KVM
uses the return value of KVM_CHECK_EXTENSION(KVM_CAP_MAX_VCPU_ID) as
the maximum APIC ID.
8. Other capabilities.
======================

6
arch/x86/include/asm/kvm_host.h
View File

@ -1243,6 +1243,12 @@ struct kvm_arch {
hpa_t hv_root_tdp;
spinlock_t hv_root_tdp_lock;
#endif
/*
* VM-scope maximum vCPU ID. Used to determine the size of structures
* that increase along with the maximum vCPU ID, in which case, using
* the global KVM_MAX_VCPU_IDS may lead to significant memory waste.
*/
u32 max_vcpu_ids;
};
struct kvm_vm_stat {

20
arch/x86/kvm/x86.c
View File

@ -6087,6 +6087,20 @@ split_irqchip_unlock:
}
mutex_unlock(&kvm->lock);
break;
case KVM_CAP_MAX_VCPU_ID:
r = -EINVAL;
if (cap->args[0] > KVM_MAX_VCPU_IDS)
break;
mutex_lock(&kvm->lock);
if (kvm->arch.max_vcpu_ids == cap->args[0]) {
r = 0;
} else if (!kvm->arch.max_vcpu_ids) {
kvm->arch.max_vcpu_ids = cap->args[0];
r = 0;
}
mutex_unlock(&kvm->lock);
break;
default:
r = -EINVAL;
break;
@ -11246,6 +11260,12 @@ int kvm_arch_vcpu_precreate(struct kvm *kvm, unsigned int id)
pr_warn_once("kvm: SMP vm created on host with unstable TSC; "
"guest TSC will not be reliable\n");
if (!kvm->arch.max_vcpu_ids)
kvm->arch.max_vcpu_ids = KVM_MAX_VCPU_IDS;
if (id >= kvm->arch.max_vcpu_ids)
return -EINVAL;
return 0;
}