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

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Date:	Thu, 6 Nov 2008 22:16:03 -0800
From:	Greg KH <greg@...ah.com>
To:	"Zhao, Yu" <yu.zhao@...el.com>
Cc:	"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

On Fri, Nov 07, 2008 at 11:40:21AM +0800, Zhao, Yu wrote:
> Greg KH wrote:
>> On Fri, Nov 07, 2008 at 10:37:55AM +0800, Zhao, Yu wrote:
>>> Greg KH wrote:
>>>> On Wed, Oct 22, 2008 at 04:45:31PM +0800, Yu Zhao wrote:
>>>>>  Documentation/kernel-parameters.txt |   10 ++++++++++
>>>>>  1 files changed, 10 insertions(+), 0 deletions(-)
>>>>>
>>>>> diff --git a/Documentation/kernel-parameters.txt 
>>>>> b/Documentation/kernel-parameters.txt
>>>>> index 53ba7c7..5482ae0 100644
>>>>> --- a/Documentation/kernel-parameters.txt
>>>>> +++ b/Documentation/kernel-parameters.txt
>>>>> @@ -1677,6 +1677,16 @@ and is between 256 and 4096 characters. It is 
>>>>> defined in the file
>>>>>  		cbmemsize=nn[KMG]	The fixed amount of bus space which is
>>>>>  				reserved for the CardBus bridge's memory
>>>>>  				window. The default value is 64 megabytes.
>>>>> +		assign-mmio=[dddd:]bb	[X86] reassign memory resources of all
>>>>> +				devices under bus [dddd:]bb (dddd is the domain
>>>>> +				number and bb is the bus number).
>>>>> +		assign-pio=[dddd:]bb	[X86] reassign io port resources of all
>>>>> +				devices under bus [dddd:]bb (dddd is the domain
>>>>> +				number and bb is the bus number).
>>>>> +		align-mmio=[dddd:]bb:dd.f  [X86] relocate memory resources of a
>>>>> +				device to minimum PAGE_SIZE alignment (dddd is
>>>>> +				the domain number and bb, dd and f is the bus,
>>>>> +				device and function number).
>>>> This seems like a big problem.  How are we going to know to add these
>>>> command line options for devices we haven't even seen/known about yet?
>>>> How do we know the bus ids aren't going to change between boots (hint,
>>>> they are, pci bus ids change all the time...)
>>>> We need to be able to do this kind of thing dynamically, not fixed at
>>>> boot time, which seems way to early to even know about this, right?
>>>> thanks,
>>>> greg k-h
>>> Yes, I totally agree. Doing things dynamically is better.
>>>
>>> The purpose of these parameters is to rebalance and align resources for 
>>> device that has BARs encapsulated in various new capabilities (SR-IOV, 
>>> etc.), because most of existing BIOSes don't take care of those BARs.
>> But how are you going to know what the proper device ids are going to
>> be before the machine boots?  I don't see how these options are ever
>> going to work properly for a "real" user.
>>> If we do resource rebalance after system is up, do you think there is any 
>>> side effect or impact to other subsystem other than PCI (e.g. MTRR)?
>> I don't think so.
>>> I haven't had much thinking on the dynamical resource rebalance. If you 
>>> have any idea about this, can you please suggest?
>> Yeah, it's going to be hard :)
>> We've thought about this in the past, and even Microsoft said it was
>> going to happen for Vista, but they realized in the end, like we did a
>> few years previously, that it would require full support of all PCI
>> drivers as well (if you rebalance stuff that is already bound to a
>> driver.)  So they dropped it.
>> When would you want to do this kind of rebalancing?  Before any PCI
>> driver is bound to any devices?  Or afterwards?
>
> I guess if we want the rebalance dynamic, then we should have it full -- 
> the rebalance would be functional even after the driver is loaded.
>
> But in most cases, there will be problem when we unload driver from a hard 
> disk controller, etc. We can mount root on a ramdisk and do the rebalance 
> there, but it's complicated for a real user.
>
> So looks like doing rebalancing before any driver is bound to any device is 
> also a nice idea, if user can get a shell to do rebalance before built-in 
> PCI driver grabs device.

That's not going to work, it needs to happen before any PCI device is
bound, which is before init runs.

thanks,

greg k-h
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