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SubjectRe: [PATCH 03/23] AMD IOMMU: implement lazy IO/TLB flushing
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On Wed, 17 Sep 2008 21:28:27 +0200
Joerg Roedel <joro@8bytes.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@amd.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@amd.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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