Subject: Re: [PATCH v2 3/3] vfio/pci: Don't probe devices that can't be reset
To: Alex Williamson <alex.williamson@redhat.com>,
        Jan Glauber <jan.glauber@caviumnetworks.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>, linux-pci@vger.kernel.org,
        linux-kernel@vger.kernel.org, david.daney@cavium.com,
        Jon Masters <jcm@redhat.com>,
        Robert Richter <robert.richter@cavium.com>,
        linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
        Jon Masters <jcm@redhat.com>
References: <1502957663-5527-1-git-send-email-jglauber@cavium.com>
 <1502957663-5527-4-git-send-email-jglauber@cavium.com>
 <20170817070017.1e9c9456@w520.home> <20170818134231.GA3464@hc>
 <20170818081251.2bbffe56@w520.home>
From: David Daney <ddaney@caviumnetworks.com>
Message-ID: <d7492153-ec84-eb87-13ac-eb3fc9116114@caviumnetworks.com>
Date: Fri, 18 Aug 2017 08:57:09 -0700
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On 08/18/2017 07:12 AM, Alex Williamson wrote:
> On Fri, 18 Aug 2017 15:42:31 +0200
> Jan Glauber <jan.glauber@caviumnetworks.com> wrote:
> 
>> On Thu, Aug 17, 2017 at 07:00:17AM -0600, Alex Williamson wrote:
>>> On Thu, 17 Aug 2017 10:14:23 +0200
>>> Jan Glauber <jglauber@cavium.com> wrote:
>>>    
>>>> If a PCI device supports neither function-level reset, nor slot
>>>> or bus reset then refuse to probe it. A line is printed to inform
>>>> the user.
>>>
>>> But that's not what this does, this requires that the device is on a
>>> reset-able bus.  This is a massive regression.  With this we could no
>>> longer assign devices on the root complex or any device which doesn't
>>> return from bus reset and currently makes use of the NO_BUS_RESET flag
>>> and works happily otherwise.  Full NAK.  Thanks,
>>
>> Looks like I missed the slot reset check. So how about this:
>>
>> if (pci_probe_reset_slot(pdev->slot) && pci_probe_reset_bus(pdev->bus)) {
>> 	dev_warn(...);
>> 	return -ENODEV;
>> }
>>
>> Or am I still missing something here?
> 
> We don't require that a device is on a reset-able bus/slot, so any
> attempt to impose that requirement means that there are devices that
> might work perfectly fine that are now excluded from assignment.  The
> entire premise is unacceptable.  Thanks,


You previously rejected the idea to silently ignore bus reset requests 
on buses that do not support it.

So this leaves us with two options:

1) Do nothing, and crash the kernel on systems with bad combinations of 
PCIe target devices and cn88xx when vfio_pci is used.

2) Do something else.

We are trying to figure out what that something else should be.  The 
general concept we are working on is that if vfio_pci wants to reset a 
device, *and* bus reset is the only option available, *and* cn88xx, then 
make vfio_pci fail.

What is your opinion of doing that (assuming it is properly implemented)?

Thanks,
David Daney