linux-kernel - Re: [PATCH] remove fullflush and nofullflush in IOMMU generic option

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Message-ID: <20080922190144.GK24392@amd.com>
Date:	Mon, 22 Sep 2008 21:01:44 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] remove fullflush and nofullflush in IOMMU generic option

On Mon, Sep 22, 2008 at 08:48:22PM +0200, Ingo Molnar wrote:
> applied Fujita's patch below to tip/x86/iommu that reverts those 
> options, thanks guys!
> 
> i'm wondering how we should proceed. For debug, iommu=off is certainly 
> good enough - all kernels are supposed to work out of box on all hw, and 
> every other result is a regression that must be fixed.
> 
> For performance tuning, it probably makes sense for developers to tune 
> IOMMU details (so that the bootup defaults can be improved - this is not 
> really something that should be done on a workload basis), but for that 
> IMO a /debug interface is a lot more useful than rather intrusive (and 
> inflexible) boot options.
>
> What do you think about a debugfs based tuning of various IOMMU details? 
> Maybe even the active driver could be changed - say from AMD-IOMMU to 
> GART, or to off.

Yes, for the IO/TLB flushing policy a /debug interface is usefull and
certainly for a number of other parameters we have today at the
commandline.
But I am not sure if we can change the IOMMU implementation on-the-fly
at runtime. We have to wait until all drivers have released their dma
memory before we can switch it off. For coherent allocations this may
take a long time.
So for selecting the type of IOMMU active in the system the command line
parameters are needed, I think. I prefer the iommu=$type parameter for
selecting the default (better: to select a default different from what
the kernel had chosen itself). We can have the IOMMU specific options to
switch it off in addition if we like to.

Joerg

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