linux-kernel - Re: [PATCH V5 3/4] x86/PCI: Stop enabling ECS for AMD CPUs after Fam16h

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Message-ID: <CAErSpo7=0-xdnu4DQufu_6eS=6AYhnDRaQvzeiMEC5a=zKU0Jg@mail.gmail.com>
Date:	Thu, 22 May 2014 14:20:04 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
	Robert Richter <rric@...nel.org>,
	Daniel J Blueman <daniel@...ascale.com>,
	Andreas Herrmann <herrmann.der.user@...glemail.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>,
	Myron Stowe <myron.stowe@...hat.com>
Subject: Re: [PATCH V5 3/4] x86/PCI: Stop enabling ECS for AMD CPUs after Fam16h

On Thu, May 22, 2014 at 1:17 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Thu, May 22, 2014 at 11:56:03AM -0600, Bjorn Helgaas wrote:
>> I chose Fam16h (0x16) because it looks like that's the newest stuff
>> that's in the field. I suspect things would probably work if we
>> changed this patch to leave ECS disabled on some Fam16h, Fam15h, etc.,
>> but that would change behavior on existing systems, which obviously
>> adds some risk. I didn't think there was much benefit that makes the
>> risk worthwhile.
>>
>> My goal is to stop needing CPU-specific changes in the future, not
>> necessarily to remove the CPU-specific code we already have.
>>
>> Does that make sense? I'm not sure whether I understood your real
>> question.
>
> No, you got it right. I'm just wondering why only the newest stuff.
> MMCONFIG is supposed to work just fine on everything from Fam10h
> onwards, I'm not sure all Fam10h supported it though. Maybe Suravee can
> verify that...

Even if MMCONFIG does work fine on everything from Fam10h onwards, we
still depend on the BIOS to provide a correct MCFG table.  I don't
think we can guarantee that changing from ECS to MMCONFIG on a Fam16h
box in the field is safe, because we'd then be using a feature we've
never used before.

Bjorn
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