linux-kernel - Re: [PATCH 1/2] KVM: X86: TSCDEADLINE MSR emulation fastpath

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Date:   Tue, 21 Apr 2020 13:37:07 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Haiwei Li <lihaiwei@...cent.com>
Subject: Re: [PATCH 1/2] KVM: X86: TSCDEADLINE MSR emulation fastpath

On 21/04/20 13:20, Wanpeng Li wrote:
> +	case MSR_IA32_TSCDEADLINE:
> +		if (!kvm_x86_ops.event_needs_reinjection(vcpu)) {
> +			data = kvm_read_edx_eax(vcpu);
> +			if (!handle_fastpath_set_tscdeadline(vcpu, data))
> +				ret = EXIT_FASTPATH_CONT_RUN;
> +		}
>  		break;

Can you explain the event_needs_reinjection case?  Also, does this break
AMD which does not implement the callback?

> +
> +	reg = kvm_lapic_get_reg(apic, APIC_LVTT);
> +	if (kvm_apic_hw_enabled(apic) && !(reg & APIC_LVT_MASKED)) {
> +		vector = reg & APIC_VECTOR_MASK;
> +		kvm_lapic_clear_vector(vector, apic->regs + APIC_TMR);
> +
> +		if (vcpu->arch.apicv_active) {
> +			if (pi_test_and_set_pir(vector, &vmx->pi_desc))
> +				return;
> +
> +			if (pi_test_and_set_on(&vmx->pi_desc))
> +				return;
> +
> +			vmx_sync_pir_to_irr(vcpu);
> +		} else {
> +			kvm_lapic_set_irr(vector, apic);
> +			kvm_queue_interrupt(vcpu, kvm_cpu_get_interrupt(vcpu), false);
> +			vmx_inject_irq(vcpu);
> +		}
> +	}

This is mostly a copy of

               if (kvm_x86_ops.deliver_posted_interrupt(vcpu, vector)) {
                        kvm_lapic_set_irr(vector, apic);
                        kvm_make_request(KVM_REQ_EVENT, vcpu);
                        kvm_vcpu_kick(vcpu);
                }
                break;

(is it required to do vmx_sync_pir_to_irr?).  So you should not special
case LVTT and move this code to lapic.c instead.  But even before that...

> 
> +
> +	if (kvm_start_hv_timer(apic)) {
> +		if (kvm_check_request(KVM_REQ_PENDING_TIMER, vcpu)) {
> +			if (kvm_x86_ops.interrupt_allowed(vcpu)) {
> +				kvm_clear_request(KVM_REQ_PENDING_TIMER, vcpu);
> +				kvm_x86_ops.fast_deliver_interrupt(vcpu);
> +				atomic_set(&apic->lapic_timer.pending, 0);
> +				apic->lapic_timer.tscdeadline = 0;
> +				return 0;
> +			}
> +			return 1;


Is it actually common that the timer is set back in time and therefore
this code is executed?

Paolo