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

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Date:   Wed, 26 Apr 2017 23:59:36 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Shaohua Li <shli@...com>
Cc:     linux-kernel@...r.kernel.org, gang.wei@...el.com,
        hpa@...ux.intel.com, mingo@...nel.org, 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

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.