linux-kernel - Re: [PATCH v7 10/39] KVM: x86: hyper-v: Don't use sparse_set_to_vcpu_mask() in kvm_hv_send

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Message-ID: <17a2e85a-a1f2-99e1-fc69-1baed2275bd5@redhat.com>
Date:   Tue, 21 Jun 2022 14:58:51 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Maxim Levitsky <mlevitsk@...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>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 10/39] KVM: x86: hyper-v: Don't use
 sparse_set_to_vcpu_mask() in kvm_hv_send_ipi()

On 6/13/22 15:38, Vitaly Kuznetsov wrote:
> Get rid of on-stack allocation of vcpu_mask and optimize kvm_hv_send_ipi()
> for a smaller number of vCPUs in the request. When Hyper-V TLB flush
> is in  use, HvSendSyntheticClusterIpi{,Ex} calls are not commonly used to
> send IPIs to a large number of vCPUs (and are rarely used in general).
> 
> Introduce hv_is_vp_in_sparse_set() to directly check if the specified
> VP_ID is present in sparse vCPU set.
> 
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>   arch/x86/kvm/hyperv.c | 37 ++++++++++++++++++++++++++-----------
>   1 file changed, 26 insertions(+), 11 deletions(-)

I'm a bit confused by this patch being in this series.

Just to be clear, PV IPI does *not* support the VP_ID, right?  And since 
patch 12 only affects the sparse banks, the patch does not have any 
other dependency.  Is this correct?

Paolo

> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index f41153c71beb..269a5fcca31b 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1747,6 +1747,25 @@ static void sparse_set_to_vcpu_mask(struct kvm *kvm, u64 *sparse_banks,
>   	}
>   }
>   
> +static bool hv_is_vp_in_sparse_set(u32 vp_id, u64 valid_bank_mask, u64 sparse_banks[])
> +{
> +	int bank, sbank = 0;
> +
> +	if (!test_bit(vp_id / HV_VCPUS_PER_SPARSE_BANK,
> +		      (unsigned long *)&valid_bank_mask))
> +		return false;
> +
> +	for_each_set_bit(bank, (unsigned long *)&valid_bank_mask,
> +			 KVM_HV_MAX_SPARSE_VCPU_SET_BITS) {
> +		if (bank == vp_id / HV_VCPUS_PER_SPARSE_BANK)
> +			break;
> +		sbank++;
> +	}
> +
> +	return test_bit(vp_id % HV_VCPUS_PER_SPARSE_BANK,
> +			(unsigned long *)&sparse_banks[sbank]);
> +}
> +
>   struct kvm_hv_hcall {
>   	u64 param;
>   	u64 ingpa;
> @@ -2029,8 +2048,8 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
>   		((u64)hc->rep_cnt << HV_HYPERCALL_REP_COMP_OFFSET);
>   }
>   
> -static void kvm_send_ipi_to_many(struct kvm *kvm, u32 vector,
> -				 unsigned long *vcpu_bitmap)
> +static void kvm_hv_send_ipi_to_many(struct kvm *kvm, u32 vector,
> +				    u64 *sparse_banks, u64 valid_bank_mask)
>   {
>   	struct kvm_lapic_irq irq = {
>   		.delivery_mode = APIC_DM_FIXED,
> @@ -2040,7 +2059,10 @@ 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))
>   			continue;
>   
>   		/* We fail only when APIC is disabled */
> @@ -2053,7 +2075,6 @@ 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;
> @@ -2115,13 +2136,7 @@ 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);
>   
>   ret_success:
>   	return HV_STATUS_SUCCESS;