Date: Wed, 26 Apr 2017 23:59:36 +0200
From: Joerg Roedel <jroedel@suse.de>
To: Shaohua Li <shli@fb.com>
Cc: linux-kernel@vger.kernel.org, gang.wei@intel.com, hpa@linux.intel.com,
        mingo@kernel.org, kernel-team@fb.com, ning.sun@intel.com,
        srihan@fb.com, alex.eydelberg@intel.com
Subject: Re: [PATCH V2] x86/tboot: add an option to disable iommu force on
Message-ID: <20170426215936.GT5077@suse.de>
References: <1c2cadcf5cd7d19cea93c56435610e61b551bd1e.1493223474.git.shli@fb.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1c2cadcf5cd7d19cea93c56435610e61b551bd1e.1493223474.git.shli@fb.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1128
Lines: 21

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@fb.com>

Applied, thanks.