Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 28 Oct 2021 16:12:04 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Maxim Levitsky <mlevitsk@redhat.com>
Cc:     Marc Zyngier <maz@kernel.org>, Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Anup Patel <anup.patel@wdc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Atish Patra <atish.patra@wdc.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-mips@vger.kernel.org, kvm@vger.kernel.org,
        kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        David Matlack <dmatlack@google.com>,
        Oliver Upton <oupton@google.com>,
        Jing Zhang <jingzhangos@google.com>
Subject: Re: [PATCH v2 27/43] KVM: VMX: Move Posted Interrupt ndst
 computation out of write loop
Message-ID: <YXrL1EuzZtTR4J1Q@google.com>
References: <20211009021236.4122790-1-seanjc@google.com>
 <20211009021236.4122790-28-seanjc@google.com>
 <643d9c249b5863f04290a6f047ea1a2d98bd75f9.camel@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <643d9c249b5863f04290a6f047ea1a2d98bd75f9.camel@redhat.com>
Precedence: bulk

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.


#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);