Re: [PATCH] vfio/pci: Support error recovery

[Cao jin <caoj.fnst@cn.fujitsu.com>]

On 12/04/2016 11:30 PM, Alex Williamson wrote:
> On Sun, 4 Dec 2016 20:16:42 +0800
> Cao jin <caoj.fnst@cn.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?

> Please go read the previous discussions, especially if you're sending
> patches you don't believe in.  Thanks,
> 

I does not read ALL version's discussion thoroughly, but these
restrictions exist for a long time, so I guess it is a result of
previous discussions.  If it is not, I am thinking of the possibility of
dropping these restrictions[2][3], and drop the "aer" property,
automatically enable this functionality or not according to device's
capability.

-- 
Sincerely,
Cao jin