Re: [PATCH v6 7/7] KVM: x86: hyperv: implement PV IPI send hypercalls

[Roman Kagan]

On Wed, Sep 26, 2018 at 07:02:59PM +0200, Vitaly Kuznetsov wrote:
> Using hypercall for sending IPIs is faster because this allows to specify
> any number of vCPUs (even > 64 with sparse CPU set), the whole procedure
> will take only one VMEXIT.
> 
> Current Hyper-V TLFS (v5.0b) claims that HvCallSendSyntheticClusterIpi
> hypercall can't be 'fast' (passing parameters through registers) but
> apparently this is not true, Windows always uses it as 'fast' so we need
> to support that.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  Documentation/virtual/kvm/api.txt |   7 ++
>  arch/x86/kvm/hyperv.c             | 115 ++++++++++++++++++++++++++++++
>  arch/x86/kvm/trace.h              |  42 +++++++++++
>  arch/x86/kvm/x86.c                |   1 +
>  include/uapi/linux/kvm.h          |   1 +
>  5 files changed, 166 insertions(+)
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 647f94128a85..1659b75d577d 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -4772,3 +4772,10 @@ CPU when the exception is taken. If this virtual SError is taken to EL1 using
>  AArch64, this value will be reported in the ISS field of ESR_ELx.
>  
>  See KVM_CAP_VCPU_EVENTS for more details.
> +8.20 KVM_CAP_HYPERV_SEND_IPI
> +
> +Architectures: x86
> +
> +This capability indicates that KVM supports paravirtualized Hyper-V IPI send
> +hypercalls:
> +HvCallSendSyntheticClusterIpi, HvCallSendSyntheticClusterIpiEx.
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index cc0535a078f7..4b4a6d015ade 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1405,6 +1405,107 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *current_vcpu, u64 ingpa,
>  		((u64)rep_cnt << HV_HYPERCALL_REP_COMP_OFFSET);
>  }
>  
> +static u64 kvm_hv_send_ipi(struct kvm_vcpu *current_vcpu, u64 ingpa, u64 outgpa,
> +			   bool ex, bool fast)
> +{
> +	struct kvm *kvm = current_vcpu->kvm;
> +	struct kvm_hv *hv = &kvm->arch.hyperv;
> +	struct hv_send_ipi_ex send_ipi_ex;
> +	struct hv_send_ipi send_ipi;
> +	struct kvm_vcpu *vcpu;
> +	unsigned long valid_bank_mask;
> +	u64 sparse_banks[64];
> +	int sparse_banks_len, bank, i, sbank;
> +	struct kvm_lapic_irq irq = {.delivery_mode = APIC_DM_FIXED};
> +	bool all_cpus;
> +
> +	if (!ex) {
> +		if (!fast) {
> +			if (unlikely(kvm_read_guest(kvm, ingpa, &send_ipi,
> +						    sizeof(send_ipi))))
> +				return HV_STATUS_INVALID_HYPERCALL_INPUT;
> +			sparse_banks[0] = send_ipi.cpu_mask;
> +			irq.vector = send_ipi.vector;
> +		} else {
> +			/* 'reserved' part of hv_send_ipi should be 0 */
> +			if (unlikely(ingpa >> 32 != 0))
> +				return HV_STATUS_INVALID_HYPERCALL_INPUT;
> +			sparse_banks[0] = outgpa;
> +			irq.vector = (u32)ingpa;
> +		}
> +		all_cpus = false;
> +		valid_bank_mask = BIT_ULL(0);
> +
> +		trace_kvm_hv_send_ipi(irq.vector, sparse_banks[0]);
> +	} else {
> +		if (unlikely(kvm_read_guest(kvm, ingpa, &send_ipi_ex,
> +					    sizeof(send_ipi_ex))))
> +			return HV_STATUS_INVALID_HYPERCALL_INPUT;
> +
> +		trace_kvm_hv_send_ipi_ex(send_ipi_ex.vector,
> +					 send_ipi_ex.vp_set.format,
> +					 send_ipi_ex.vp_set.valid_bank_mask);
> +
> +		irq.vector = send_ipi_ex.vector;
> +		valid_bank_mask = send_ipi_ex.vp_set.valid_bank_mask;
> +		sparse_banks_len = bitmap_weight(&valid_bank_mask, 64) *
> +			sizeof(sparse_banks[0]);
> +
> +		all_cpus = send_ipi_ex.vp_set.format == HV_GENERIC_SET_ALL;
> +
> +		if (!sparse_banks_len)
> +			goto ret_success;
> +
> +		if (!all_cpus &&
> +		    kvm_read_guest(kvm,
> +				   ingpa + offsetof(struct hv_send_ipi_ex,
> +						    vp_set.bank_contents),
> +				   sparse_banks,
> +				   sparse_banks_len))
> +			return HV_STATUS_INVALID_HYPERCALL_INPUT;
> +	}
> +
> +	if ((irq.vector < HV_IPI_LOW_VECTOR) ||
> +	    (irq.vector > HV_IPI_HIGH_VECTOR))
> +		return HV_STATUS_INVALID_HYPERCALL_INPUT;
> +
> +	if (all_cpus || atomic_read(&hv->num_mismatched_vp_indexes)) {
> +		kvm_for_each_vcpu(i, vcpu, kvm) {
> +			if (all_cpus || hv_vcpu_in_sparse_set(
> +				    &vcpu->arch.hyperv, sparse_banks,
> +				    valid_bank_mask)) {
> +				/* We fail only when APIC is disabled */
> +				kvm_apic_set_irq(vcpu, &irq, NULL);
> +			}
> +		}
> +		goto ret_success;
> +	}
> +
> +	/*
> +	 * num_mismatched_vp_indexes is zero so every vcpu has
> +	 * vp_index == vcpu_idx.
> +	 */
> +	sbank = 0;
> +	for_each_set_bit(bank, (unsigned long *)&valid_bank_mask, 64) {
> +		for_each_set_bit(i, (unsigned long *)&sparse_banks[sbank], 64) {
> +			u32 vp_index = bank * 64 + i;
> +			struct kvm_vcpu *vcpu =
> +				get_vcpu_by_vpidx(kvm, vp_index);
> +
> +			/* Unknown vCPU specified */
> +			if (!vcpu)
> +				continue;
> +
> +			/* We fail only when APIC is disabled */
> +			kvm_apic_set_irq(vcpu, &irq, NULL);
> +		}
> +		sbank++;
> +	}
> +
> +ret_success:
> +	return HV_STATUS_SUCCESS;
> +}
> +

I must say that now it looks even more tempting to follow the same
pattern as your kvm_hv_flush_tlb: define a function that would call
kvm_apic_set_irq() on all vcpus in a mask (optimizing the all-set case
with a NULL mask), and make kvm_hv_send_ipi perform the same hv_vp_set
-> vcpu_mask transformation followed by calling into that function.

Roman.