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

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Date:   Fri, 7 Apr 2017 12:08:40 +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: [RFC] x86/tboot: add an option to disable iommu force on

On Mon, Apr 03, 2017 at 12:19:28PM -0700, Shaohua Li wrote:
> On Wed, Mar 22, 2017 at 07:50:55AM -0400, Shaohua Li wrote:
> > On Wed, Mar 22, 2017 at 11:49:00AM +0100, Joerg Roedel wrote:
> > > Hi Shaohua,
> > > 
> > > On Tue, Mar 21, 2017 at 11:37:51AM -0700, Shaohua Li wrote:
> > > > IOMMU harms performance signficantly when we run very fast networking
> > > > workloads. 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.
> > > 
> > > Can you elaborate a bit more on the setup where the IOMMU still harms
> > > network performance? With the recent scalability improvements I measured
> > > only a minimal impact on 10GBit networking.
> > Hi,
> > 
> > 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.
> 
> Any update on this?

An explicit Ack from the tboot guys would be good to have.


	Joerg