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

[Cao jin]

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.
-- 
Sincerely,
Cao jin