Re: [PATCH v4 4/8] KVM: Optimize kvm_make_vcpus_request_mask() a bit

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 27, 2021, Vitaly Kuznetsov wrote:
> Iterating over set bits in 'vcpu_bitmap' should be faster than going
> through all vCPUs, especially when just a few bits are set.
> 
> Drop kvm_make_vcpus_request_mask() call from kvm_make_all_cpus_request_except()
> to avoid handling the special case when 'vcpu_bitmap' is NULL, move the
> code to kvm_make_all_cpus_request_except() itself.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  virt/kvm/kvm_main.c | 88 +++++++++++++++++++++++++++------------------
>  1 file changed, 53 insertions(+), 35 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 2082aceffbf6..e32ba210025f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -261,50 +261,57 @@ static inline bool kvm_kick_many_cpus(cpumask_var_t tmp, bool wait)
>  	return true;
>  }
>  
> +static void kvm_make_vcpu_request(struct kvm *kvm, struct kvm_vcpu *vcpu,
> +				  unsigned int req, cpumask_var_t tmp,
> +				  int current_cpu)
> +{
> +	int cpu = vcpu->cpu;

'cpu' doesn't need to be initialized here.  Leaving it uninitialized will also
deter consumption before the READ_ONCE below.

> +
> +	kvm_make_request(req, vcpu);
> +
> +	if (!(req & KVM_REQUEST_NO_WAKEUP) && kvm_vcpu_wake_up(vcpu))
> +		return;
> +
> +	/*
> +	 * tmp can be "unavailable" if cpumasks are allocated off stack as
> +	 * allocation of the mask is deliberately not fatal and is handled by
> +	 * falling back to kicking all online CPUs.
> +	 */
> +	if (!cpumask_available(tmp))
> +		return;
> +
> +	/*
> +	 * Note, the vCPU could get migrated to a different pCPU at any point
> +	 * after kvm_request_needs_ipi(), which could result in sending an IPI
> +	 * to the previous pCPU.  But, that's OK because the purpose of the IPI
> +	 * is to ensure the vCPU returns to OUTSIDE_GUEST_MODE, which is
> +	 * satisfied if the vCPU migrates. Entering READING_SHADOW_PAGE_TABLES
> +	 * after this point is also OK, as the requirement is only that KVM wait
> +	 * for vCPUs that were reading SPTEs _before_ any changes were
> +	 * finalized. See kvm_vcpu_kick() for more details on handling requests.
> +	 */
> +	if (kvm_request_needs_ipi(vcpu, req)) {
> +		cpu = READ_ONCE(vcpu->cpu);
> +		if (cpu != -1 && cpu != current_cpu)
> +			__cpumask_set_cpu(cpu, tmp);
> +	}
> +}