linux-kernel - Re: [PATCH V2] x86/tboot: add an option to disable iommu force on

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Date:   Thu, 27 Apr 2017 08:52:53 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Shaohua Li <shli@...com>, linux-kernel@...r.kernel.org,
        gang.wei@...el.com, hpa@...ux.intel.com, kernel-team@...com,
        ning.sun@...el.com, srihan@...com, alex.eydelberg@...el.com
Subject: Re: [PATCH V2] x86/tboot: add an option to disable iommu force on


* Joerg Roedel <jroedel@...e.de> wrote:

> On Wed, Apr 26, 2017 at 09:18:35AM -0700, Shaohua Li wrote:
> > IOMMU harms performance signficantly when we run very fast networking
> > workloads. It's 40GB networking doing XDP test. Software overhead is
> > almost unaware, but it's the IOTLB miss (based on our analysis) which
> > kills the performance. We observed the same performance issue even with
> > software passthrough (identity mapping), only the hardware passthrough
> > survives. The pps with iommu (with software passthrough) is only about
> > ~30% of that without it. This is a limitation in hardware based on our
> > observation, so we'd like to disable the IOMMU force on, but we do want
> > to use TBOOT and we can sacrifice the DMA security bought by IOMMU. I
> > must admit I know nothing about TBOOT, but TBOOT guys (cc-ed) think not
> > eabling IOMMU is totally ok.
> > 
> > So introduce a new boot option to disable the force on. It's kind of
> > silly we need to run into intel_iommu_init even without force on, but we
> > need to disable TBOOT PMR registers. For system without the boot option,
> > nothing is changed.
> > 
> > Signed-off-by: Shaohua Li <shli@...com>
> 
> Applied, thanks.

Please don't apply it yet, I posted a few review questions.

Thanks,

	Ingo