linux-kernel - Re: [Patch v2] Make PCI extended config space (MMCONFIG) a driver opt-in

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Message-ID: <478A8020.4000203@myri.com>
Date:	Sun, 13 Jan 2008 16:18:24 -0500
From:	Loic Prylli <loic@...i.com>
To:	Matthew Wilcox <matthew@....cx>
CC:	Arjan van de Ven <arjan@...radead.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Greg KH <greg@...ah.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	Jeff Garzik <jeff@...zik.org>,
	linux-pci@...ey.karlin.mff.cuni.cz,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Martin Mares <mj@....cz>, Tony Camuso <tcamuso@...hat.com>
Subject: Re: [Patch v2] Make PCI extended config space (MMCONFIG) a driver
 opt-in

On 1/13/2008 3:43 PM, Matthew Wilcox wrote:
> On Sun, Jan 13, 2008 at 10:41:24AM -0800, Arjan van de Ven wrote:
>   
>> Note: There is not a 100% overlap between "need" and "will not be used in 
>> the patches that use legacy for < 256". In the other patches posted, 
>> extended config space will be used in cases where it won't be with my 
>> patch. (Most obvious one is an "lspci -vx" from automated scripts). 
>>     
>
> I believe you to be mistaken in this belief.  If you take Ivan's patch,
> conf1 is used for all accesses below 256 bytes.  lspci -x only dumps
> config space up to 64 bytes; lspci -xxxx is needed to show extended pci
> config space.
>   

I agree with Arjan about that "not a 100% overlap". It is about the 
extra ext-conf-space access done while probing in drivers/pci/probe.c:
    dev->cfg_size = pci_cfg_space_size(dev);

(and lspci -v will also query/show the list of extended-caps for 
pci-x/pcie-x devices that have some, provided the kernel can access 
ext-conf-space).

With Ivan's patch, that line would still cause one extended-conf-space 
access at offset 256 for pcie/pci-x2 devices  (to check the ability to 
query ext-space). Arjan "opt-in" patch would prevent that extra access.

IMHO that access is OK and harmless in all cases, we are already 
protected by MCFG/e820 checks, but I agree one can express a different 
opinion based on trying to prevent "never-seen/potential" hardware/BIOS 
bugs. FWIW it is also there that I was suggested to exclude PCI-X2 
devices (when restricted to pcie, that access while probing cannot even 
cause the harmless master-abort/0xffffffff), but there is a small trade-off.

Loic

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