RE: [RFC] x86/tboot: add an option to disable iommu force on

[Sun, Ning]

Hi Shaohua,

One question, did you still see the network performance penalty when Linux kernel cmdline intel_iommu was set to off ( intel_iommu=off) ?

Thanks,
-ning

-----Original Message-----
From: Joerg Roedel [mailto:jroedel@xxxxxxx] 
Sent: Friday, April 07, 2017 3:09 AM
To: Shaohua Li <shli@xxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx; Wei, Gang <gang.wei@xxxxxxxxx>; hpa@xxxxxxxxxxxxxxx; mingo@xxxxxxxxxx; kernel-team@xxxxxx; Sun, Ning <ning.sun@xxxxxxxxx>; srihan@xxxxxx; Eydelberg, Alex <alex.eydelberg@xxxxxxxxx>
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