linux-kernel - Re: [PATCH v4 5/8] KVM: introduce kvm_make_vcpus_request

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Date:   Sat, 26 May 2018 16:49:47 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/8] KVM: introduce kvm_make_vcpus_request_mask() API

2018-05-16 17:21+0200, Vitaly Kuznetsov:
> Hyper-V style PV TLB flush hypercalls inmplementation will use this API.
> To avoid memory allocation in CONFIG_CPUMASK_OFFSTACK case add
> cpumask_var_t argument.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> -bool kvm_make_all_cpus_request(struct kvm *kvm, unsigned int req)
> +bool kvm_make_vcpus_request_mask(struct kvm *kvm, unsigned int req,
> +				 unsigned long *vcpu_bitmap, cpumask_var_t tmp)
>  {
>  	int i, cpu, me;
> -	cpumask_var_t cpus;
> -	bool called;
>  	struct kvm_vcpu *vcpu;
> -
> -	zalloc_cpumask_var(&cpus, GFP_ATOMIC);
> +	bool called;
>  
>  	me = get_cpu();
> +

Two optimizations come to mind: First is to use for_each_set_bit instead
of kvm_for_each_vcpu to improve the sparse case.

>  	kvm_for_each_vcpu(i, vcpu, kvm) {
> +		if (!test_bit(i, vcpu_bitmap))

And the second is to pass vcpu_bitmap = NULL instead of building the
bitmap with all VCPUs.  Doesn't looks too good in the end, though:

#define kvm_for_each_vcpu_bitmap(idx, vcpup, kvm, bitmap, len) \
	for (idx = (bitmap ? find_first_bit(bitmap, len) : 0); \
	     idx < len && (vcpup = kvm_get_vcpu(kvm, idx)) != NULL; \
	     bitmap ? find_next_bit(bitmap, len, idx + 1) : idx++)