linux-kernel - Re: [PATCH 03/23] AMD IOMMU: implement lazy IO/TLB flushing

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Date:	Wed, 17 Sep 2008 21:28:27 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	joerg.roedel@....com, iommu@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/23] AMD IOMMU: implement lazy IO/TLB flushing

On Thu, Sep 18, 2008 at 04:20:18AM +0900, FUJITA Tomonori wrote:
> On Wed, 17 Sep 2008 18:52:37 +0200
> Joerg Roedel <joerg.roedel@....com> wrote:
> 
> > The IO/TLB flushing on every unmaping operation is the most expensive
> > part there and not strictly necessary. It is sufficient to do the flush
> > before any entries are reused. This is patch implements lazy IO/TLB
> > flushing which does exactly this.
> > 
> > Signed-off-by: Joerg Roedel <joerg.roedel@....com>
> > ---
> >  Documentation/kernel-parameters.txt |    5 +++++
> >  arch/x86/kernel/amd_iommu.c         |   26 ++++++++++++++++++++++----
> >  arch/x86/kernel/amd_iommu_init.c    |   10 +++++++++-
> >  include/asm-x86/amd_iommu_types.h   |    9 +++++++++
> >  4 files changed, 45 insertions(+), 5 deletions(-)
> > 
> > diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> > index c2e00ee..5f0aefe 100644
> > --- a/Documentation/kernel-parameters.txt
> > +++ b/Documentation/kernel-parameters.txt
> > @@ -284,6 +284,11 @@ and is between 256 and 4096 characters. It is defined in the file
> >  			isolate - enable device isolation (each device, as far
> >  			          as possible, will get its own protection
> >  			          domain)
> > +			unmap_flush - enable flushing of IO/TLB entries when
> > +			              they are unmapped. Otherwise they are
> > +				      flushed before they will be reused, which
> > +				      is a lot of faster
> > +
> 
> Would it be nice to have consistency of IOMMU parameters?

True. We should merge common parameters across IOMMUs into the
iommu= parameter some time in the future, I think. It would also be the
place for the IOMMU size parameter.

> VT-d also has the kernel-boot option for this lazy flushing trick
> though VT-d 'strict' option is more vague than 'unmap_flush'
> 
> It would be also nice to have consistency of IOMMU behavior.
> 
> VT-d enables the lazy flushing trick by default and has the boot
> option to disable it.

This is exactly what AMD IOMMU with this patch does too. The
amd_iommu=unmap_flush parameter disables lazy IO/TLB flushing.


Joerg
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