linux-kernel - Re: Workaround for Intel MPS errata

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Message-ID: <4E8B04D8.5010107@redhat.com>
Date:	Tue, 04 Oct 2011 15:06:32 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jon Mason <mason@...i.com>
CC:	Sven Schnelle <svens@...ckframe.org>, Simon Kirby <sim@...tway.ca>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Niels Ole Salscheider <niels_ole@...scheider-online.de>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Ben Hutchings <bhutchings@...arflare.com>
Subject: Re: Workaround for Intel MPS errata

On 10/04/2011 11:46 AM, Avi Kivity wrote:
> On 10/03/2011 05:12 PM, Jon Mason wrote:
>>      PCI: Workaround for Intel MPS errata
>>
>>      Intel 5000 and 5100 series memory controllers have a known issue 
>> if read
>>      completion coalescing is enabled (the default setting) and the 
>> PCI-E
>>      Maximum Payload Size is set to 256B.  To work around this issue, 
>> disable
>>      read completion coalescing if the MPS is 256B.
>>
>>      It is worth noting that there is no function to undo the disable 
>> of read
>>      completion coalescing, and the performance benefit of read 
>> completion
>>      coalescing will be lost if the MPS is set from 256B to 128B.  It 
>> is only
>>      possible to have this issue via hotplug removing the only 256B MPS
>>      device in the system (thus making all of the other devices in 
>> the system
>>      have a performance degradation without the benefit of any 256B
>>      transfers).  Therefore, this trade off is acceptable.
>>
>>      
>> http://www.intel.com/content/dam/doc/specification-update/5000-chipset-memory-controller-hub-specification-update.pdf
>>      
>> http://www.intel.com/content/dam/doc/specification-update/5100-memory-controller-hub-chipset-specification-update.pdf
>>
>>      Thanks to Jesse Brandeburg and Ben Hutchings for providing 
>> insight into
>>      the problem.
>>
>>      Reported-by: Avi Kivity<avi@...hat.com>
>>      Signed-off-by: Jon Mason<mason@...i.com>
>>
>> +
>> +        if (!(val&  (1<<  10))) {
>> +            done = true;
>> +            return;
>> +        }
>
> Here, you bail out if bit 10 is clear.  So if we're here, it's set.
>
>> +
>> +        val |= (1<<  10);
>
> Now it's even more set?
>

Even with this line changed to clear bit 10, I still get a hard lockup.  
Do we need to clear this bit on the other 5000 devices?  I notice they 
have similar values in word 0x48, with bits 10 set in them.

What does "Device 7-2,0" refer to in the workaround description?  Seems 
to me we need to apply the workaround to the PCIe ports as well.

-- 
error compiling committee.c: too many arguments to function

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