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

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Date:	Thu, 18 Sep 2008 10:29:24 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	joro@...tes.org
Cc:	fujita.tomonori@....ntt.co.jp, 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 Wed, 17 Sep 2008 21:28:27 +0200
Joerg Roedel <joro@...tes.org> wrote:

> 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.

Hmm, now is better than the future? I think that now you can add
something like 'disable_batching_flush' as a common parameter and
change AMD IOMMU to use it.


> > 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.

Oops, I misread the description.
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