[PATCH 4.19 02/34] KVM: remember position in kvm->vcpus array

Greg Kroah-Hartman
2021-09-24T12:50:51Z

From: Radim Krčmář <rkrcmar@redhat.com>

commit 8750e72a79dda2f665ce17b62049f4d62130d991 upstream.

Fetching an index for any vcpu in kvm->vcpus array by traversing
the entire array everytime is costly.
This patch remembers the position of each vcpu in kvm->vcpus array
by storing it in vcpus_idx under kvm_vcpu structure.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[borntraeger@de.ibm.com]: backport to 4.19 (also fits for 5.4)
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/kvm_host.h | 11 +++--------
virt/kvm/kvm_main.c | 5 +++--
2 files changed, 6 insertions(+), 10 deletions(-)

--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -248,7 +248,8 @@ struct kvm_vcpu {
struct preempt_notifier preempt_notifier;
#endif
int cpu;
- int vcpu_id;
+ int vcpu_id; /* id given by userspace at creation */
+ int vcpu_idx; /* index in kvm->vcpus array */
int srcu_idx;
int mode;
u64 requests;
@@ -551,13 +552,7 @@ static inline struct kvm_vcpu *kvm_get_v

static inline int kvm_vcpu_get_idx(struct kvm_vcpu *vcpu)
{
- struct kvm_vcpu *tmp;
- int idx;
-
- kvm_for_each_vcpu(idx, tmp, vcpu->kvm)
- if (tmp == vcpu)
- return idx;
- BUG();
+ return vcpu->vcpu_idx;
}

#define kvm_for_each_memslot(memslot, slots) \
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2751,7 +2751,8 @@ static int kvm_vm_ioctl_create_vcpu(stru
goto unlock_vcpu_destroy;
}

- BUG_ON(kvm->vcpus[atomic_read(&kvm->online_vcpus)]);
+ vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
+ BUG_ON(kvm->vcpus[vcpu->vcpu_idx]);

/* Now it's all set up, let userspace reach it */
kvm_get_kvm(kvm);
@@ -2761,7 +2762,7 @@ static int kvm_vm_ioctl_create_vcpu(stru
goto unlock_vcpu_destroy;
}

- kvm->vcpus[atomic_read(&kvm->online_vcpus)] = vcpu;
+ kvm->vcpus[vcpu->vcpu_idx] = vcpu;

/*
* Pairs with smp_rmb() in kvm_get_vcpu. Write kvm->vcpus