linux-kernel - Re: [PATCH v2 27/43] KVM: VMX: Move Posted Interrupt ndst computation out of write loop

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Message-ID: <432666f36add6647283631770f1b140656c67c62.camel@redhat.com>
Date:   Mon, 01 Nov 2021 00:51:39 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v2 27/43] KVM: VMX: Move Posted Interrupt ndst
 computation out of write loop

On Thu, 2021-10-28 at 16:12 +0000, Sean Christopherson wrote:
> On Thu, Oct 28, 2021, Maxim Levitsky wrote:
> > On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:
> > > Hoist the CPU => APIC ID conversion for the Posted Interrupt descriptor
> > > out of the loop to write the descriptor, preemption is disabled so the
> > > CPU won't change, and if the APIC ID changes KVM has bigger problems.
> > > 
> > > No functional change intended.
> > 
> > Is preemption always disabled in vmx_vcpu_pi_load? vmx_vcpu_pi_load is called
> > from vmx_vcpu_load, which is called indirectly from vcpu_load which is called
> > from many ioctls, which userspace does. In these places I don't think that
> > preemption is disabled.
> 
> Preemption is disabled in vcpu_load() by the get_cpu().  The "cpu" param that's
> passed around the vcpu_load() stack is also why I think it's ok to _not_ assert
> that preemption is disabled in vmx_vcpu_pi_load(); if preemption is enabled,
> "cpu" is unstable and thus the entire "load" operation is busted.

Yes, I even knew about the get_cpu() behavier which indeed has to disable preemption.
But I didn't notice call to it, when I wrote this mail! Later I did notice it but it was
too late. Sometimes sending all the review mails at once at the end does make sense after all,
I guess.

Best regards,
	Maxim Levitsky

> 
> 
> #define get_cpu()		({ preempt_disable(); __smp_processor_id(); })
> #define put_cpu()		preempt_enable()
> 
> 
> void vcpu_load(struct kvm_vcpu *vcpu)
> {
> 	int cpu = get_cpu();
> 
> 	__this_cpu_write(kvm_running_vcpu, vcpu);
> 	preempt_notifier_register(&vcpu->preempt_notifier);
> 	kvm_arch_vcpu_load(vcpu, cpu);
> 	put_cpu();
> }
> EXPORT_SYMBOL_GPL(vcpu_load);
>