linux-kernel - Re: [PATCH 2/2] KVM: VMX: Handle preemption timer fastpath

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Message-ID: <68eb0e46-4c2a-0292-3dfa-db2ae2b2b13d@redhat.com>
Date:   Tue, 21 Apr 2020 13:40:06 +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 2/2] KVM: VMX: Handle preemption timer fastpath

On 21/04/20 13:20, Wanpeng Li wrote:
> +
> +	if (!vmx->req_immediate_exit &&
> +		!unlikely(vmx->loaded_vmcs->hv_timer_soft_disabled)) {
> +		if (!vmx_interrupt_allowed(vcpu) ||
> +			!apic_lvtt_tscdeadline(apic) ||
> +			vmx->rmode.vm86_active ||
> +			is_smm(vcpu) ||
> +			!kvm_apic_hw_enabled(apic))
> +			return EXIT_FASTPATH_NONE;
> +
> +		if (!apic->lapic_timer.hv_timer_in_use)
> +			return EXIT_FASTPATH_CONT_RUN;
> +
> +		WARN_ON(swait_active(&vcpu->wq));
> +		vmx_cancel_hv_timer(vcpu);
> +		apic->lapic_timer.hv_timer_in_use = false;
> +
> +		if (atomic_read(&apic->lapic_timer.pending))
> +			return EXIT_FASTPATH_CONT_RUN;
> +
> +		ktimer->expired_tscdeadline = ktimer->tscdeadline;
> +		vmx_fast_deliver_interrupt(vcpu);
> +		ktimer->tscdeadline = 0;
> +		return EXIT_FASTPATH_CONT_RUN;
> +	}
> +

Can you explain all the checks you have here, and why you need something
more complex than apic_timer_expired (possibly by adding some
optimizations to kvm_apic_local_deliver)?  This code is impossible to
maintain.

Paolo