Message-ID: <4E5D3CD4.8@linux.vnet.ibm.com>
Date:	Tue, 30 Aug 2011 14:41:08 -0500
From:	Brian King <brking@...ux.vnet.ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Jan Kiszka <jan.kiszka@...mens.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Brian King <brking@...ibm.com>,
	"James E.J. Bottomley" <JBottomley@...allels.com>,
	"Hans J. Koch" <hjk@...sjkoch.de>,
	Greg Kroah-Hartman <gregkh@...e.de>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: Broken pci_block_user_cfg_access interface

On 08/30/2011 01:01 PM, Michael S. Tsirkin wrote:
> On Tue, Aug 30, 2011 at 11:30:35AM -0500, Brian King wrote:
>> On 08/29/2011 02:18 PM, Michael S. Tsirkin wrote:
>>> On Mon, Aug 29, 2011 at 08:47:07PM +0200, Jan Kiszka wrote:
>>>> On 2011-08-29 17:42, Jan Kiszka wrote:
>>>>> I still don't get what prevents converting ipr to allow plain mutex
>>>>> synchronization. My vision is:
>>>>>  - push reset-on-error of ipr into workqueue (or threaded IRQ?)
>>>>
>>>> I'm starting to like your proposal: I had a look at ipr, but it turned
>>>> out to be anything but trivial to convert that driver. It runs its
>>>> complete state machine under spin_lock_irq, and the functions calling
>>>> pci_block/unblock_user_cfg_access are deep inside this thing. I have no
>>>> hardware to test whatever change, and I feel a bit uncomfortable asking
>>>> Brian to redesign his driver that massively.
>>>>
>>>> So back to your idea: I would generalize pci_block_user_cfg_access to
>>>> pci_block_cfg_access. It should fail when some other site already holds
>>>> the access lock, but it should remain non-blocking - for the sake of ipr.
>>>
>>> It would be easy to have blocking and non-blocking variants.
>>>
>>> But
>>> - I have no idea whether supporting sysfs config/reset access
>>>   while ipr is active makes any sense - I know we need it for uio.
>>
>> I really don't think it makes sense. Ideally, I really think the driver
>> should be able to override the PCI layer reset interface in sysfs.
> 
> Well, it's always possible for root to do silly things
> like reset the device from userspace using sysfs config
> access. So I don't really see this blocking as necessary:
> broken application with access to a physical device will lead
> to problems.

I agree that it is probably not necessary for the current use of the sysfs API,
but if there was interest in expanding it to be usable for other purposes,
we might need to do something like what I am proposing. However, it doesn't
appear that anyone has any immediate need for doing so.

> 
> 
>> If a driver
>> is loaded, the driver owns all the state of that device and resetting it without
>> informing the driver is just nasty. Additionally, many devices may have
>> much more complex logic to performing a reset than what PCI defines. With
>> ipr, for example, it needs to get a shutdown command issued to it prior to the
>> reset if at all possible so that the firmware quiesces any I/O it is performing.
>> It also needs additional communication prior to resetting the chip to ensure
>> the firmware is not modifying its persistent error log on the adapter's flash,
>> since resetting the card while the flash segment is being updated will cause
>> the adapter to lose the persistent error log. Post reset, ipr has a bunch
>> of work to do to get the firmware back up and running to a state where it
>> can handle I/O again.
>>
>> Different ipr chips also have different requirements as to what
>> reset mechanisms defined by PCI actually work. Some chips require BIST to be
>> run via PCI config space, while others require a PCI warm reset, otherwise
>> the card ends up in an unusable state.
>>
>> So, here is my proposal to resolve this particular issue.
> 
> As userspace has no place touching the device while
> ipr is active, there's no issue with ipr at all, is there?

Correct.


>> Add a reset function
>> to the pci_driver struct which would allow drivers to override the default reset
>> action. Drivers that can tolerate the existing reset mechanism can simply point
>> to a generic PCI function to perform the reset. Drivers which can't handle their
>> device getting reset, will simply not have a reset function defined. In this case,
>> anyone attempting to issue a reset via sysfs will get an error. If a driver
>> is not loaded, then we can perform the default reset method and fix any device
>> specific oddities with quirks.
>>
>> I like keeping pci_block_user_cfg_access a non blocking interface. If it can
>> return a failure due to some other caller, it should be easy enough to modify
>> the ipr driver to wait for access to get unblocked before resetting the adapter.
>>
>> Thanks,
>>
>> Brian
> 
> There shouldn't be other callers though, should there?
> So it might be enough to fail gracefully (to make debugging
> easier) rather than retry.

Actually, its probably not worth the complexity to change much of anything in the
ipr driver. With the current users, ipr should be the only caller. If, for some
reason, we were double blocked due to ipr, the ipr driver already handles that today
just fine and having the second block essentially be a noop is fine, since we will
only get a single unblock due to ipr's reset state machine. If another caller is
added in the future, then we will need to make potential changes, but without knowing
the use case, its not clear to me what the proper action would be anyway.

Thanks,

Brian

-- 
Brian King
Linux on Power Virtualization
IBM Linux Technology Center


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