netdev - Re: [PATCH V2 2/6] kvm: detect assigned device via irqbypass manager

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Message-ID: <20200717120821.3c2a56db@x1.home>
Date:   Fri, 17 Jul 2020 12:08:21 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Zhu Lingshan <lingshan.zhu@...el.com>
Cc:     mst@...hat.com, pbonzini@...hat.com,
        sean.j.christopherson@...el.com, wanpengli@...cent.com,
        jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH V2 2/6] kvm: detect assigned device via irqbypass
 manager

On Thu, 16 Jul 2020 19:23:45 +0800
Zhu Lingshan <lingshan.zhu@...el.com> wrote:

> vDPA devices has dedicated backed hardware like
> passthrough-ed devices. Then it is possible to setup irq
> offloading to vCPU for vDPA devices. Thus this patch tries to
> manipulated assigned device counters via irqbypass manager.
> 
> We will increase/decrease the assigned device counter in kvm/x86.
> Both vDPA and VFIO would go through this code path.
> 
> This code path only affect x86 for now.
> 
> Signed-off-by: Zhu Lingshan <lingshan.zhu@...el.com>
> Suggested-by: Jason Wang <jasowang@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 00c88c2..20c07d3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10624,11 +10624,17 @@ int kvm_arch_irq_bypass_add_producer(struct irq_bypass_consumer *cons,
>  {
>  	struct kvm_kernel_irqfd *irqfd =
>  		container_of(cons, struct kvm_kernel_irqfd, consumer);
> +	int ret;
>  
>  	irqfd->producer = prod;
> +	kvm_arch_start_assignment(irqfd->kvm);
> +	ret = kvm_x86_ops.update_pi_irte(irqfd->kvm,
> +					 prod->irq, irqfd->gsi, 1);
> +
> +	if (ret)
> +		kvm_arch_end_assignment(irqfd->kvm);
>  
> -	return kvm_x86_ops.update_pi_irte(irqfd->kvm,
> -					   prod->irq, irqfd->gsi, 1);
> +	return ret;
>  }
>  
>  void kvm_arch_irq_bypass_del_producer(struct irq_bypass_consumer *cons,


Why isn't there a matching end-assignment in the del_producer path?  It
seems this only goes one-way, what happens when a device is
hot-unplugged from the VM or the device interrupt configuration changes.
This will still break vfio if it's not guaranteed to be symmetric.
Thanks,

Alex