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

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Date:	Thu, 6 Nov 2008 18:50:32 -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 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?

thanks,

greg k-h
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