Date:   Tue, 6 Dec 2016 14:11:03 +0800
From:   Cao jin <caoj.fnst@...fujitsu.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <izumi.taku@...fujitsu.com>, <mst@...hat.com>
Subject: Re: [PATCH] vfio/pci: Support error recovery



On 12/06/2016 12:17 AM, Alex Williamson wrote:
> On Mon, 5 Dec 2016 13:52:03 +0800
> Cao jin <caoj.fnst@...fujitsu.com> wrote:
> 
>> On 12/04/2016 11:30 PM, Alex Williamson wrote:
>>> On Sun, 4 Dec 2016 20:16:42 +0800
>>> Cao jin <caoj.fnst@...fujitsu.com> wrote:
>>>   
>>>> On 12/01/2016 10:55 PM, Alex Williamson wrote:  
>>>>> On Thu, 1 Dec 2016 21:40:00 +0800    
>>>>  
>>>>>>> If an AER fault occurs and the user doesn't do a reset, what
>>>>>>> happens when that device is released and a host driver tries to make
>>>>>>> use of it?  The user makes no commitment to do a reset and there are
>>>>>>> only limited configurations where we even allow the user to perform a
>>>>>>> reset.
>>>>>>>       
>>>>>>
>>>>>> Limited? Do you mean the things __pci_dev_reset() can do?    
>>>>>
>>>>> I mean that there are significant device and guest configuration
>>>>> restrictions in order to support AER.  For instance, all the functions
>>>>> of the slot need to appear in a PCI-e topology in the guest with all
>>>>> the functions in the right place such that a guest bus reset translates
>>>>> into a host bus reset.  The physical functions cannot be split between
>>>>> guests even if IOMMU isolation would otherwise allow it.  The user
>>>>> needs to explicitly enable AER support for the devices.  A VM need to
>>>>> be specifically configured for AER support in order to set any sort of
>>>>> expectations of a guest directed bus reset, let alone a guarantee that
>>>>> it will happen.  So all the existing VMs, where functions are split
>>>>> between guests, or the topology isn't exactly right, or AER isn't
>>>>> enabled see a regression from the above change as the device is no
>>>>> longer reset.
>>>>>     
>>>>
>>>> I am not clear why set these restrictions in the current design. I take
>>>> a glance at older versions of qemu's patchset, their thoughts is:
>>>> translate a guest bus reset into a host bus reset(Which is
>>>> unreasonable[*] to me). And I guess, that's the *cause* of these
>>>> restrictions?  Is there any other stories behind these restrictions?
>>>>
>>>> [*] In physical world, set bridge's secondary bus reset would send
>>>> hot-reset TLP to all functions below, trigger every device's reset
>>>> separately. Emulated device should behave the same, means just using
>>>> each device's DeviceClass->reset method.  
>>>
>>> Are you trying to say that an FLR is equivalent to a link reset?  
>>
>> No.  Look at old versions patchset, there is one names "vote the
>> function 0 to do host bus reset when aer occurred"[1], that is what I
>> called "translate guest link reset to host link reset", and what I think
>> unreasonable(and I think it also does it wrongly).  So in v10 version of
>> mine, I dropped it.
>>
>> [1]https://lists.gnu.org/archive/html/qemu-devel/2016-05/msg02987.html
>>
>> If "translate guest link reset to host link reset" is right, I can
>> understand these restrictions[2][3].
>>
>> [2]. All physical functions in a single card must be assigned to the VM
>>      with AER enabled on each and configured on the same virtual bus.
>> [3]. Don't place other devices under the virtual bus in [2], no matter
>>      physical, emulated, or paravirtual, even if other device
>>      supporting AER signaling
>>
>> Certain device's FLR calls its DeviceClass->reset method; link reset
>> calls DeviceClass->reset of each device which on the bus. So, apparently
>> they have difference.  But if there is only 1 vfio-pci device under the
>> virtual pci bus,  I think FLR can be equivalent to a link reset, right?
> 
> No.  An FLR resets the device while a secondary bus reset does a reset
> of the link and the device.  AER errors are sometimes issues with the
> link, not the device.  If we were to perform only an FLR, we're not
> performing the same reset as would be done on bare metal.
>  

Thanks for you explanation, it does helps, except the last sentence, I
think I understand it now: fatal error implies there may be link issue
exists(pci express spec: 6.2.2.2.1), so, should do link reset for fatal
error(that is what and why aer core does). And so, in patch[1] above,
qemu does a link reset when seeing secondary bus reset bit of virtual
bus got set. is it right?

-- 
Sincerely,
Cao jin

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