Re: [PATCH v4 7/8] KVM: Pre-allocate cpumasks for kvm_make_all_cpus_request_except()

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 27, 2021, Vitaly Kuznetsov wrote:
> Allocating cpumask dynamically in zalloc_cpumask_var() is not ideal.
> Allocation is somewhat slow and can (in theory and when CPUMASK_OFFSTACK)
> fail. kvm_make_all_cpus_request_except() already disables preemption so
> we can use pre-allocated per-cpu cpumasks instead.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  virt/kvm/kvm_main.c | 29 +++++++++++++++++++++++------
>  1 file changed, 23 insertions(+), 6 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 2e9927c4eb32..2f5fe4f54a51 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -155,6 +155,8 @@ static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm);
>  static unsigned long long kvm_createvm_count;
>  static unsigned long long kvm_active_vms;
>  
> +static DEFINE_PER_CPU(cpumask_var_t, cpu_kick_mask);
> +
>  __weak void kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
>  						   unsigned long start, unsigned long end)
>  {
> @@ -323,14 +325,15 @@ bool kvm_make_all_cpus_request_except(struct kvm *kvm, unsigned int req,
>  				      struct kvm_vcpu *except)
>  {
>  	struct kvm_vcpu *vcpu;
> -	cpumask_var_t cpus;
> +	struct cpumask *cpus;
>  	bool called;
>  	int i, me;
>  
> -	zalloc_cpumask_var(&cpus, GFP_ATOMIC);
> -
>  	me = get_cpu();
>  
> +	cpus = this_cpu_cpumask_var_ptr(cpu_kick_mask);
> +	cpumask_clear(cpus);
> +
>  	kvm_for_each_vcpu(i, vcpu, kvm) {
>  		if (vcpu == except)
>  			continue;
> @@ -340,7 +343,6 @@ bool kvm_make_all_cpus_request_except(struct kvm *kvm, unsigned int req,
>  	called = kvm_kick_many_cpus(cpus, !!(req & KVM_REQUEST_WAIT));
>  	put_cpu();
>  
> -	free_cpumask_var(cpus);
>  	return called;
>  }
>  
> @@ -5581,9 +5583,15 @@ int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
>  		goto out_free_3;
>  	}
>  
> +	for_each_possible_cpu(cpu) {
> +		if (!alloc_cpumask_var_node(&per_cpu(cpu_kick_mask, cpu),
> +					    GFP_KERNEL, cpu_to_node(cpu)))
> +			goto out_free_4;

'r' needs to be explicitly set to -EFAULT, e.g. in the current code it's
guaranteed to be 0 here.

> +	}
> +
>  	r = kvm_async_pf_init();
>  	if (r)
> -		goto out_free;
> +		goto out_free_5;
>  
>  	kvm_chardev_ops.owner = module;
>  	kvm_vm_fops.owner = module;
> @@ -5609,7 +5617,11 @@ int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
>  
>  out_unreg:
>  	kvm_async_pf_deinit();
> -out_free:
> +out_free_5:
> +	for_each_possible_cpu(cpu) {

Unnecessary braces.

> +		free_cpumask_var(per_cpu(cpu_kick_mask, cpu));
> +	}
> +out_free_4:
>  	kmem_cache_destroy(kvm_vcpu_cache);
>  out_free_3:
>  	unregister_reboot_notifier(&kvm_reboot_notifier);
> @@ -5629,8 +5641,13 @@ EXPORT_SYMBOL_GPL(kvm_init);
>  
>  void kvm_exit(void)
>  {
> +	int cpu;
> +
>  	debugfs_remove_recursive(kvm_debugfs_dir);
>  	misc_deregister(&kvm_dev);
> +	for_each_possible_cpu(cpu) {

Same here.

> +		free_cpumask_var(per_cpu(cpu_kick_mask, cpu));
> +	}
>  	kmem_cache_destroy(kvm_vcpu_cache);
>  	kvm_async_pf_deinit();
>  	unregister_syscore_ops(&kvm_syscore_ops);
> -- 
> 2.31.1
>