Enabling CONFIG_GART_IOMMU by default?

[Josh Boyer]

On Sun, May 3, 2015 at 5:25 AM, Tomáš Trnka <tomastrnka at gmx.com> wrote:
> Hello,
>
> I have a PCI-based SATA controller (Sil3124) that chokes on PCI DAC
> addressing, so I have to force 32-bit addressing through an IOMMU. Sadly, the
> stock Fedora kernel falls back to SWIOTLB on this machine, which is both
> slower (though I haven't benchmarked it) and unreliable (several times I've
> seen the SWIOTLB overflow and go boom).
>
> Apparently, all I need is the AMD GART-based IOMMU implementation
> (CONFIG_GART_IOMMU) that is currently disabled by Fedora. This means I
> currently have to rebuild the kernel every time to re-enable it.

GART_IOMMU is the really old IOMMU found in older Athlon64/Opteron
CPUs.  Most modern AMD CPUS use AMD_IOMMU, which is enabled.  Do you
have one of these older CPUs?

> Is there any particular reason why this is disabled in Fedora (upstream
> Kconfig default is enabled)? Given that there is probably a non-negligible
> fraction of machines that could benefit from the GART IOMMU and the kernel
> size impact is minimal, would it be possible to restore the upstream default?

Upstream doesn't include it in the x86_64_defconfig and it isn't
default y.  The Kconfig help text says to enable if unsure, but if you
look at the commit log where this option was last changed, it's pretty
discouraging.

josh

commit 38901f1c1cae241fd4e3c5e63fcc690b921734b1
Author: Andi Kleen <ak at linux.intel.com>
Date:   Fri Oct 4 14:37:56 2013 -0700

    x86/iommu: Don't make AMD_GART depend on EXPERT and default y

    The AMD_GART driver was made EXPERT/EMBEDDED a long time
    ago to avoid unbootable 64bit systems with 32bit only devices.

    This was before swiotlb was there, which does the job
    of this fallback today. SWIOTLB is always on, so systems
    should always boot.

    The drawback is that every system has to compile that
    driver in (it cannot be a module).

    Also:
     - Newer AMD CPUs (the APUs) don't seem to have AMD_GART support
       at all anymore.

     - Newer AMD platforms have a much better real IOMMU

     - The AMD GART driver was never very good (lots of overhead,
       e.g. in flushing due to some workarounds) and it's doubtful it's
       really better than SWIOTLB.

     - On older K8 systems it didn't even work with all chipsets.

     - The 32bit device bounce buffer case should be rare/
       non performance critical these days anyways.

     - On non AMD systems it is not needed at all.

    So drop the EXPERT dependency on AMD_GART and remove the
    default y. The driver can be still compiled in, just
    it's an explicit decision now, and people who don't want
    it can unselect it.

    I also clarified the description a bit.

    This allows to save ~8K text on most modern x86-64 systems.

    Signed-off-by: Andi Kleen <ak at linux.intel.com>
    Acked-by: Borislav Petkov <bp at suse.de>
    Link: http://lkml.kernel.org/r/1380922676-23007-1-git-send-email-andi@firstfloor.org
    Signed-off-by: Ingo Molnar <mingo at kernel.org>