linux-kernel - Re: [PATCH v2 06/31] KVM: x86: hyper-v: Don't use sparse_set_to_vcpu_mask() in kvm_hv_send

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Message-ID: <Yk8j8pTnnFMymyds@google.com>
Date:   Thu, 7 Apr 2022 17:48:34 +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>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 06/31] KVM: x86: hyper-v: Don't use
 sparse_set_to_vcpu_mask() in kvm_hv_send_ipi()

On Thu, Apr 07, 2022, 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.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/hyperv.c | 35 ++++++++++++++++++++++++-----------
>  1 file changed, 24 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index d7bcdf87b90c..918642bcdbd0 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1746,6 +1746,23 @@ 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 / 64, (unsigned long *)&valid_bank_mask))

'64' really, really, really needs a #define.  I assume this is the same '64' that's
used to check the var_cnt when getting the sparse_banks.

> +		return false;
> +
> +	for_each_set_bit(bank, (unsigned long *)&valid_bank_mask,
> +			 KVM_HV_MAX_SPARSE_VCPU_SET_BITS) {
> +		if (bank == vp_id / 64)
> +			break;
> +		sbank++;
> +	}
> +
> +	return test_bit(vp_id % 64, (unsigned long *)&sparse_banks[sbank]);
> +}