linux-kernel - Re: [Question] Indefinitely block in the host when remove the PF driver

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Date:   Tue, 11 May 2021 11:44:49 +0800
From:   Yicong Yang <yangyicong@...ilicon.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        <qemu-devel@...gnu.org>
CC:     <cohuck@...hat.com>, <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Zengtao (B)" <prime.zeng@...ilicon.com>,
        Linuxarm <linuxarm@...wei.com>
Subject: Re: [Question] Indefinitely block in the host when remove the PF
 driver

[ +qemu-devel ]

On 2021/4/30 22:29, Alex Williamson wrote:
> On Fri, 30 Apr 2021 15:57:47 +0800
> Yicong Yang <yangyicong@...ilicon.com> wrote:
> 
>> When I try to remove the PF driver in the host, the process will be blocked
>> if the related VF of the device is added in the Qemu as an iEP.
>>
>> here's what I got in the host:
>>
>> [root@...alhost 0000:75:00.0]# rmmod hisi_zip
>> [99760.571352] vfio-pci 0000:75:00.1: Relaying device request to user (#0)
>> [99862.992099] vfio-pci 0000:75:00.1: Relaying device request to user (#10)
>> [...]
>>
>> and in the Qemu:
>>
>> estuary:/$ lspci -tv
>> -[0000:00]-+-00.0  Device 1b36:0008
>>            +-01.0  Device 1af4:1000
>>            +-02.0  Device 1af4:1009
>>            \-03.0  Device 19e5:a251 <----- the related VF device
>> estuary:/$ qemu-system-aarch64: warning: vfio 0000:75:00.1: Bus 'pcie.0' does not support hotplugging
>> qemu-system-aarch64: warning: vfio 0000:75:00.1: Bus 'pcie.0' does not support hotplugging
>> qemu-system-aarch64: warning: vfio 0000:75:00.1: Bus 'pcie.0' does not support hotplugging
>> qemu-system-aarch64: warning: vfio 0000:75:00.1: Bus 'pcie.0' does not support hotplugging
>> [...]
>>
>> The rmmod process will be blocked until I kill the Qemu process. That's the only way if I
>> want to end the rmmod.
>>
>> So my question is: is such block reasonable? If the VF devcie is occupied or doesn't
>> support hotplug in the Qemu, shouldn't we fail the rmmod and return something like -EBUSY
>> rather than make the host blocked indefinitely?
> 
> Where would we return -EBUSY?  pci_driver.remove() returns void.
> Without blocking, I think our only option would be to kill the user
> process.
>  

yes. the remove() callback of pci_driver doesn't provide a way to abort the process.

>> Add the VF under a pcie root port will avoid this. Is it encouraged to always
>> add the VF under a pcie root port rather than directly add it as an iEP?
> 
> Releasing a device via the vfio request interrupt is always a
> cooperative process currently, the VM needs to be configured such that
> the device is capable of being unplugged and the guest needs to respond
> to the ejection request.  Thanks,
> 

Does it make sense to abort the VM creation and give some warnings if user try to
pass a vfio pci device to the Qemu and doesn't attach it to a hotpluggable
bridge? Currently I think there isn't such a mechanism in Qemu.

Thanks,
Yicong