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Subject: Re: [PATCH 1/4] KVM: arm/arm64: vgic: Do not cond_resched_lock() with
 IRQs disabled
To:     Christoffer Dall <christoffer.dall@arm.com>
Cc:     linux-kernel@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
        marc.zyngier@arm.com, linux-arm-kernel@lists.infradead.org,
        linux-rt-users@vger.kernel.org, tglx@linutronix.de,
        rostedt@goodmis.org, bigeasy@linutronix.de, stable@vger.kernel.org
References: <1542647279-46609-1-git-send-email-julien.thierry@arm.com>
 <1542647279-46609-2-git-send-email-julien.thierry@arm.com>
 <20181120141832.GA11162@e113682-lin.lund.arm.com>
From:   Julien Thierry <julien.thierry@arm.com>
Message-ID: <76a488a9-893c-ffbe-16f2-23735af8126a@arm.com>
Date:   Tue, 20 Nov 2018 17:22:39 +0000
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On 20/11/18 14:18, Christoffer Dall wrote:
> On Mon, Nov 19, 2018 at 05:07:56PM +0000, Julien Thierry wrote:
>> To change the active state of an MMIO, halt is requested for all vcpus of
>> the affected guest before modifying the IRQ state. This is done by calling
>> cond_resched_lock() in vgic_mmio_change_active(). However interrupts are
>> disabled at this point and running a vcpu cannot get rescheduled.
> 
> "running a vcpu cannot get rescheduled" ?

Yes, that doesn't make much sense :\ . I guess I just meant "a vcpu 
cannot get rescheduled on this cpu".

I'll rewrite this.

> 
>>
>> Solve this by waiting for all vcpus to be halted after emmiting the halt
>> request.
>>
>> Fixes commit 6c1b7521f4a07cc63bbe2dfe290efed47cdb780a ("KVM: arm/arm64:
>> Factor out functionality to get vgic mmio requester_vcpu")
>>
>> Signed-off-by: Julien Thierry <julien.thierry@arm.com>
>> Suggested-by: Marc Zyngier <marc.zyngier@arm.com>
>> Cc: Christoffer Dall <christoffer.dall@arm.com>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> Cc: stable@vger.kernel.org
>> ---
>>   virt/kvm/arm/vgic/vgic-mmio.c | 33 +++++++++++----------------------
>>   1 file changed, 11 insertions(+), 22 deletions(-)
>>
>> diff --git a/virt/kvm/arm/vgic/vgic-mmio.c b/virt/kvm/arm/vgic/vgic-mmio.c
>> index f56ff1c..eefd877 100644
>> --- a/virt/kvm/arm/vgic/vgic-mmio.c
>> +++ b/virt/kvm/arm/vgic/vgic-mmio.c
>> @@ -313,27 +313,6 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,
>>
>>   	spin_lock_irqsave(&irq->irq_lock, flags);
>>
>> -	/*
>> -	 * If this virtual IRQ was written into a list register, we
>> -	 * have to make sure the CPU that runs the VCPU thread has
>> -	 * synced back the LR state to the struct vgic_irq.
>> -	 *
>> -	 * As long as the conditions below are true, we know the VCPU thread
>> -	 * may be on its way back from the guest (we kicked the VCPU thread in
>> -	 * vgic_change_active_prepare)  and still has to sync back this IRQ,
>> -	 * so we release and re-acquire the spin_lock to let the other thread
>> -	 * sync back the IRQ.
>> -	 *
>> -	 * When accessing VGIC state from user space, requester_vcpu is
>> -	 * NULL, which is fine, because we guarantee that no VCPUs are running
>> -	 * when accessing VGIC state from user space so irq->vcpu->cpu is
>> -	 * always -1.
>> -	 */
>> -	while (irq->vcpu && /* IRQ may have state in an LR somewhere */
>> -	       irq->vcpu != requester_vcpu && /* Current thread is not the VCPU thread */
>> -	       irq->vcpu->cpu != -1) /* VCPU thread is running */
>> -		cond_resched_lock(&irq->irq_lock);
>> -
>>   	if (irq->hw) {
>>   		vgic_hw_irq_change_active(vcpu, irq, active, !requester_vcpu);
>>   	} else {
>> @@ -368,8 +347,18 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,
>>    */
>>   static void vgic_change_active_prepare(struct kvm_vcpu *vcpu, u32 intid)
>>   {
>> -	if (intid > VGIC_NR_PRIVATE_IRQS)
>> +	if (intid > VGIC_NR_PRIVATE_IRQS) {
>> +		struct kvm_vcpu *tmp;
>> +		int i;
>> +
>>   		kvm_arm_halt_guest(vcpu->kvm);
>> +
>> +		/* Wait for each vcpu to be halted */
>> +		kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
>> +			while (tmp->cpu != -1)
>> +				cond_resched();
> 
> We used to have something like this which Andre then found out it could
> deadlock the system, because the VCPU making this request wouldn't have
> called kvm_arch_vcpu_put, and its cpu value would still have a value.
> 
> That's why we have the vcpu && vcpu != requester check.
> 

I see, thanks for pointing that out. I'll fix this in the next iteration.

Thanks,

-- 
Julien Thierry