linux-kernel - Re: [PATCH] intel-iommu: Default to non-coherent for domains unattached to iommus

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Date:	Mon, 14 Nov 2011 23:12:11 -0800
From:	cody <mail.kai.huang@...il.com>
To:	David Woodhouse <dwmw2@...radead.org>
CC:	Chris Wright <chrisw@...s-sol.org>,
	Roland Dreier <roland@...estorage.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, ddutile@...hat.com
Subject: Re: [PATCH] intel-iommu: Default to non-coherent for domains unattached
 to iommus

On 11/11/2011 05:20 PM, David Woodhouse wrote:
> On Fri, 2011-11-11 at 17:08 -0800, Chris Wright wrote:
>    
>> All the stale PTE issues I've encountered in the past have turned into
>> fixed sw bugs (perhaps it's since been fixed?).  Also, I thought with
>> Coherency On/Off it's only effecting the use of clflush, not IOTLB or
>> Context Entry cache flushing (invalidations).
>>      
> Yeah, it's supposed to be *just* clflush. Nevertheless, I can imagine it
> being screwed up and there actually being a buffer in the chipset too.
> We certainly made that mistake with the graphics engine in some cases...
>
>    
>> On a slightly separate, but performance related note...have you ever
>> tried using the hw queue?  Currently we only have a sw queue, but the
>> submission path for invalidations doesn't really queue (unless I missed
>> it).  It seems to pull from the software queue and submit/wait,
>> submit/wait...Seems simple enough to submit the whole queue and then
>> issue the wait.
>>      
> I have a feeling we trigger errata if we do that — although if we're
> only doing it for an emulated IOMMU that shouldn't be an issue.
>
>    
What does the emulated IOMMU here? Does it mean the emulated IOMMU 
exposed to guest VM?

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