Subject: Re: [PATCH v3 19/41] KVM: PPC: Book3S HV P9: Stop handling hcalls in real-mode in the P9 path

From: Fabiano Rosas <[email-masked]>
Date: Wed, 17 Mar 2021 13:22:37 -0300

Nicholas Piggin <[email-masked]> writes:

> In the interest of minimising the amount of code that is run in
> "real-mode", don't handle hcalls in real mode in the P9 path.
>
> POWER8 and earlier are much more expensive to exit from HV real mode
> and switch to host mode, because on those processors HV interrupts get
> to the hypervisor with the MMU off, and the other threads in the core
> need to be pulled out of the guest, and SLBs all need to be saved,
> ERATs invalidated, and host SLB reloaded before the MMU is re-enabled
> in host mode. Hash guests also require a lot of hcalls to run. The
> XICS interrupt controller requires hcalls to run.
>
> By contrast, POWER9 has independent thread switching, and in radix mode
> the hypervisor is already in a host virtual memory mode when the HV
> interrupt is taken. Radix + xive guests don't need hcalls to handle
> interrupts or manage translations.
>
> So it's much less important to handle hcalls in real mode in P9.
>
> Signed-off-by: Nicholas Piggin <[email-masked]>
> ---

<snip>

> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 497f216ad724..1f2ba8955c6a 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -1147,7 +1147,7 @@ int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
>   * This has to be done early, not in kvmppc_pseries_do_hcall(), so
>   * that the cede logic in kvmppc_run_single_vcpu() works properly.
>   */
> -static void kvmppc_nested_cede(struct kvm_vcpu *vcpu)
> +static void kvmppc_cede(struct kvm_vcpu *vcpu)

The comment above needs to be updated I think.

>  {
>  	vcpu->arch.shregs.msr |= MSR_EE;
>  	vcpu->arch.ceded = 1;
> @@ -1403,9 +1403,15 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
>  		/* hcall - punt to userspace */
>  		int i;
>
> -		/* hypercall with MSR_PR has already been handled in rmode,
> -		 * and never reaches here.
> -		 */
> +		if (unlikely(vcpu->arch.shregs.msr & MSR_PR)) {
> +			/*
> +			 * Guest userspace executed sc 1, reflect it back as a
> +			 * privileged program check interrupt.
> +			 */
> +			kvmppc_core_queue_program(vcpu, SRR1_PROGPRIV);
> +			r = RESUME_GUEST;
> +			break;
> +		}
>
>  		run->papr_hcall.nr = kvmppc_get_gpr(vcpu, 3);
>  		for (i = 0; i < 9; ++i)
> @@ -3740,15 +3746,36 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
>  		/* H_CEDE has to be handled now, not later */
>  		if (trap == BOOK3S_INTERRUPT_SYSCALL && !vcpu->arch.nested &&
>  		    kvmppc_get_gpr(vcpu, 3) == H_CEDE) {
> -			kvmppc_nested_cede(vcpu);
> +			kvmppc_cede(vcpu);
>  			kvmppc_set_gpr(vcpu, 3, 0);
>  			trap = 0;
>  		}
>  	} else {
>  		kvmppc_xive_push_vcpu(vcpu);
>  		trap = kvmhv_load_hv_regs_and_go(vcpu, time_limit, lpcr);
> -		kvmppc_xive_pull_vcpu(vcpu);
> +		/* H_CEDE has to be handled now, not later */
> +		/* XICS hcalls must be handled before xive is pulled */
> +		if (trap == BOOK3S_INTERRUPT_SYSCALL &&
> +		    !(vcpu->arch.shregs.msr & MSR_PR)) {
> +			unsigned long req = kvmppc_get_gpr(vcpu, 3);
>
> +			if (req == H_CEDE) {
> +				kvmppc_cede(vcpu);
> +				kvmppc_xive_cede_vcpu(vcpu); /* may un-cede */
> +				kvmppc_set_gpr(vcpu, 3, 0);
> +				trap = 0;
> +			}
> +			if (req == H_EOI || req == H_CPPR ||
> +			    req == H_IPI || req == H_IPOLL ||
> +			    req == H_XIRR || req == H_XIRR_X) {
> +				unsigned long ret;
> +
> +				ret = kvmppc_xive_xics_hcall(vcpu, req);
> +				kvmppc_set_gpr(vcpu, 3, ret);
> +				trap = 0;
> +			}
> +		}

I tried running L2 with xive=off and this code slows down the boot
considerably. I think we're missing a !vcpu->arch.nested in the
conditional.

This may also be missing these checks from kvmppc_pseries_do_hcall:

		if (kvmppc_xics_enabled(vcpu)) {
			if (xics_on_xive()) {
				ret = H_NOT_AVAILABLE;
				return RESUME_GUEST;
			}
			ret = kvmppc_xics_hcall(vcpu, req);
                        (...)

For H_CEDE there might be a similar situation since we're shadowing the
code above that runs after H_ENTER_NESTED by setting trap to 0 here.

> +		kvmppc_xive_pull_vcpu(vcpu);
>  	}
>
>  	vcpu->arch.slb_max = 0;
> @@ -4408,8 +4435,11 @@ static int kvmppc_vcpu_run_hv(struct kvm_vcpu *vcpu)
>  		else
>  			r = kvmppc_run_vcpu(vcpu);
>
> -		if (run->exit_reason == KVM_EXIT_PAPR_HCALL &&
> -		    !(vcpu->arch.shregs.msr & MSR_PR)) {
> +		if (run->exit_reason == KVM_EXIT_PAPR_HCALL) {
> +			if (WARN_ON_ONCE(vcpu->arch.shregs.msr & MSR_PR)) {
> +				r = RESUME_GUEST;
> +				continue;
> +			}
>  			trace_kvm_hcall_enter(vcpu);
>  			r = kvmppc_pseries_do_hcall(vcpu);
>  			trace_kvm_hcall_exit(vcpu, r);
> diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> index c11597f815e4..2d0d14ed1d92 100644
> --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> @@ -1397,9 +1397,14 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  	mr	r4,r9
>  	bge	fast_guest_return
>  2:
> +	/* If we came in through the P9 short path, no real mode hcalls */
> +	lwz	r0, STACK_SLOT_SHORT_PATH(r1)
> +	cmpwi	r0, 0
> +	bne	no_try_real
>  	/* See if this is an hcall we can handle in real mode */
>  	cmpwi	r12,BOOK3S_INTERRUPT_SYSCALL
>  	beq	hcall_try_real_mode
> +no_try_real:
>
>  	/* Hypervisor doorbell - exit only if host IPI flag set */
>  	cmpwi	r12, BOOK3S_INTERRUPT_H_DOORBELL
> diff --git a/arch/powerpc/kvm/book3s_xive.c b/arch/powerpc/kvm/book3s_xive.c
> index 52cdb9e2660a..1e4871bbcad4 100644
> --- a/arch/powerpc/kvm/book3s_xive.c
> +++ b/arch/powerpc/kvm/book3s_xive.c
> @@ -158,6 +158,40 @@ void kvmppc_xive_pull_vcpu(struct kvm_vcpu *vcpu)
>  }
>  EXPORT_SYMBOL_GPL(kvmppc_xive_pull_vcpu);
>
> +void kvmppc_xive_cede_vcpu(struct kvm_vcpu *vcpu)
> +{
> +	void __iomem *esc_vaddr = (void __iomem *)vcpu->arch.xive_esc_vaddr;
> +
> +	if (!esc_vaddr)
> +		return;
> +
> +	/* we are using XIVE with single escalation */
> +
> +	if (vcpu->arch.xive_esc_on) {
> +		/*
> +		 * If we still have a pending escalation, abort the cede,
> +		 * and we must set PQ to 10 rather than 00 so that we don't
> +		 * potentially end up with two entries for the escalation
> +		 * interrupt in the XIVE interrupt queue.  In that case
> +		 * we also don't want to set xive_esc_on to 1 here in
> +		 * case we race with xive_esc_irq().
> +		 */
> +		vcpu->arch.ceded = 0;
> +		/*
> +		 * The escalation interrupts are special as we don't EOI them.
> +		 * There is no need to use the load-after-store ordering offset
> +		 * to set PQ to 10 as we won't use StoreEOI.
> +		 */
> +		__raw_readq(esc_vaddr + XIVE_ESB_SET_PQ_10);
> +	} else {
> +		vcpu->arch.xive_esc_on = true;
> +		mb();
> +		__raw_readq(esc_vaddr + XIVE_ESB_SET_PQ_00);
> +	}
> +	mb();
> +}
> +EXPORT_SYMBOL_GPL(kvmppc_xive_cede_vcpu);
> +
>  /*
>   * This is a simple trigger for a generic XIVE IRQ. This must
>   * only be called for interrupts that support a trigger page
> @@ -2106,6 +2140,32 @@ static int kvmppc_xive_create(struct kvm_device *dev, u32 type)
>  	return 0;
>  }
>
> +int kvmppc_xive_xics_hcall(struct kvm_vcpu *vcpu, u32 req)
> +{
> +	struct kvmppc_vcore *vc = vcpu->arch.vcore;
> +
> +	switch (req) {
> +	case H_XIRR:
> +		return xive_vm_h_xirr(vcpu);
> +	case H_CPPR:
> +		return xive_vm_h_cppr(vcpu, kvmppc_get_gpr(vcpu, 4));
> +	case H_EOI:
> +		return xive_vm_h_eoi(vcpu, kvmppc_get_gpr(vcpu, 4));
> +	case H_IPI:
> +		return xive_vm_h_ipi(vcpu, kvmppc_get_gpr(vcpu, 4),
> +					  kvmppc_get_gpr(vcpu, 5));
> +	case H_IPOLL:
> +		return xive_vm_h_ipoll(vcpu, kvmppc_get_gpr(vcpu, 4));
> +	case H_XIRR_X:
> +		xive_vm_h_xirr(vcpu);
> +		kvmppc_set_gpr(vcpu, 5, get_tb() + vc->tb_offset);
> +		return H_SUCCESS;
> +	}
> +
> +	return H_UNSUPPORTED;
> +}
> +EXPORT_SYMBOL_GPL(kvmppc_xive_xics_hcall);
> +
>  int kvmppc_xive_debug_show_queues(struct seq_file *m, struct kvm_vcpu *vcpu)
>  {
>  	struct kvmppc_xive_vcpu *xc = vcpu->arch.xive_vcpu;