linux-kernel - Re: [PATCH 16/16 v6] PCI: document the new PCI boot parameters

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Date:	Sat, 08 Nov 2008 14:05:41 +0800
From:	Yu Zhao <yu.zhao@...scape.net>
To:	Greg KH <greg@...ah.com>
CC:	"Zhao, Yu" <yu.zhao@...el.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"achiang@...com" <achiang@...com>,
	"grundler@...isc-linux.org" <grundler@...isc-linux.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	"matthew@....cx" <matthew@....cx>,
	"randy.dunlap@...cle.com" <randy.dunlap@...cle.com>,
	"rdreier@...co.com" <rdreier@...co.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH 16/16 v6] PCI: document the new PCI boot parameters

Greg KH wrote:
> On Sat, Nov 08, 2008 at 01:00:29PM +0800, Yu Zhao wrote:
>> Greg KH wrote:
>>> On Fri, Nov 07, 2008 at 04:35:47PM +0800, Zhao, Yu wrote:
>>>> Greg KH wrote:
>>>>> On Fri, Nov 07, 2008 at 04:17:02PM +0800, Zhao, Yu wrote:
>>>>>>> Well, to do it "correctly" you are going to have to tell the driver to
>>>>>>> shut itself down, and reinitialize itself.
>>>>>>> Turns out, that doesn't really work for disk and network devices 
>>>>>>> without
>>>>>>> dropping the connection (well, network devices should be fine 
>>>>>>> probably).
>>>>>>> So you just can't do this, sorry.  That's why the BIOS handles all of
>>>>>>> these issues in a PCI hotplug system.
>>>>>>> How does the hardware people think we are going to handle this in the
>>>>>>> OS?  It's not something that any operating system can do, is it part 
>>>>>>> of
>>>>>>> the IOV PCI spec somewhere?
>>>>>> No, it's not part of the PCI IOV spec.
>>>>>>
>>>>>> I just want the IOV (and whole PCI subsystem) have more flexibility on 
>>>>>> various BIOSes. So can we reconsider about resource rebalance as boot 
>>>>>> option, or should we forget about this idea?
>>>>> As you have proposed it, the boot option will not work at all, so I
>>>>> think we need to forget about it.  Especially if it is not really
>>>>> needed.
>>>> I guess at least one thing would work if people don't want to boot twice: 
>>>> give the bus number 0 as rebalance starting point, then all system 
>>>> resources would be reshuffled :-)
>>> Hm, but don't we do that today with our basic resource reservation logic
>>> at boot time?  What would be different about this kind of proposal?
>> The generic PCI core can do this but this feature is kind of disabled by 
>> low level PCI code in x86. The low level code tries to reserve resource 
>> according to configuration from BIOS. If the BIOS is wrong, the allocation 
>> would fail and the generic PCI core couldn't repair it because the bridge 
>> resources may have been allocated by the PCI low level and the PCI core 
>> can't expand them to find enough resource for the subordinates.
> 
> Yes, we do this on purpose.
> 
>> The proposal is to disable x86 PCI low level to allocation resources 
>> according to BIOS so PCI core can fully control the resource allocation. 
>> The PCI core takes all resources from BARs it knows into account and 
>> configure the resource windows on the bridges according to its own 
>> calculation.
> 
> Ah, so you mean we should revert back to the way we use to do x86 PCI
> resource allocation from about a year and a half ago to about 8 years
> ago?
> 
> Hint, there was a reason why we switched over to using the BIOS instead
> of doing it ourselves.  Turns out we have to trust the BIOS here, as
> that is exactly what other operating systems do.  Trying to do it on our
> own was too fragile and resulted in too many problems over time.
> 
> Go look at the archives for when this all was switched, you'll see the
> reasons why.
> 
> So no, we will not be going back to the way we used to do things, we
> changed for a reason :)

So it's really a long story, and I'm glad to see the reason.

Actually there was no such thing in early SR-IOV patches, but months ago 
I heard some complaints that pushed me to do this kind of reverse. Looks 
like I have to let these complaints turn to BIOS people from now on :-)

Regards,
Yu
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