linux-kernel - Re: [PATCH v11 16/46] KVM: x86: hyper-v: Don't use sparse_set_to_vcpu_mask() in kvm_hv_send

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Message-ID: <Y1BahCzO4jxFC9Ey@google.com>
Date:   Wed, 19 Oct 2022 20:13:56 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        Yuan Yao <yuan.yao@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 16/46] KVM: x86: hyper-v: Don't use
 sparse_set_to_vcpu_mask() in kvm_hv_send_ipi()

On Tue, Oct 04, 2022, Vitaly Kuznetsov wrote:
> @@ -2034,7 +2056,10 @@ static void kvm_send_ipi_to_many(struct kvm *kvm, u32 vector,
>  	unsigned long i;
>  
>  	kvm_for_each_vcpu(i, vcpu, kvm) {
> -		if (vcpu_bitmap && !test_bit(i, vcpu_bitmap))
> +		if (sparse_banks &&
> +		    !hv_is_vp_in_sparse_set(kvm_hv_get_vpindex(vcpu),
> +					    valid_bank_mask,
> +					    sparse_banks))

Nit, this fits on two lines and IMO is slightly easier on the eyes:

		if (sparse_banks &&
		    !hv_is_vp_in_sparse_set(kvm_hv_get_vpindex(vcpu),
					    valid_bank_mask, sparse_banks))
			continue;

>  			continue;
>  
>  		/* We fail only when APIC is disabled */
> @@ -2047,7 +2072,6 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
>  	struct kvm *kvm = vcpu->kvm;
>  	struct hv_send_ipi_ex send_ipi_ex;
>  	struct hv_send_ipi send_ipi;
> -	DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
>  	u64 valid_bank_mask;
>  	u64 sparse_banks[KVM_HV_MAX_SPARSE_VCPU_SET_BITS];
>  	u32 vector;
> @@ -2109,13 +2133,7 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
>  	if ((vector < HV_IPI_LOW_VECTOR) || (vector > HV_IPI_HIGH_VECTOR))
>  		return HV_STATUS_INVALID_HYPERCALL_INPUT;
>  
> -	if (all_cpus) {
> -		kvm_send_ipi_to_many(kvm, vector, NULL);
> -	} else {
> -		sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask, vcpu_mask);
> -
> -		kvm_send_ipi_to_many(kvm, vector, vcpu_mask);
> -	}
> +	kvm_hv_send_ipi_to_many(kvm, vector, all_cpus ? NULL : sparse_banks, valid_bank_mask);

Any objection to not using a ternary operator?

	if (all_cpus)
		kvm_hv_send_ipi_to_many(kvm, vector, NULL, 0);
	else
		kvm_hv_send_ipi_to_many(kvm, vector, sparse_banks, valid_bank_mask);

Mostly because it's somewhat arbitrary that earlier code ensures valid_bank_mask
is set in the all_cpus=true case, e.g. arguably KVM doesn't need to do the var_cnt
sanity check in the all_cpus case:

		all_cpus = send_ipi_ex.vp_set.format == HV_GENERIC_SET_ALL;
		if (all_cpus)
			goto check_and_send_ipi;

		valid_bank_mask = send_ipi_ex.vp_set.valid_bank_mask;
		if (hc->var_cnt != hweight64(valid_bank_mask))
			return HV_STATUS_INVALID_HYPERCALL_INPUT;

		if (!hc->var_cnt)
			goto ret_success;