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

[Shaohua Li <shli@fb.com>]

On Fri, Apr 07, 2017 at 09:49:52PM +0000, Sun, Ning wrote:
> 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) ?

the boot parameter has no effect, it runs very early and set dmar_disable=1.
The tboot code (tboot_force_iommu) runs later and force dmar_disabled = 0.

Thanks,
Shaohua
 
> Thanks,
> -ning
> 
> -----Original Message-----
> From: Joerg Roedel [mailto:jroedel@suse.de] 
> Sent: Friday, April 07, 2017 3:09 AM
> To: Shaohua Li <shli@fb.com>
> Cc: linux-kernel@vger.kernel.org; Wei, Gang <gang.wei@intel.com>; hpa@linux.intel.com; mingo@kernel.org; kernel-team@fb.com; Sun, Ning <ning.sun@intel.com>; srihan@fb.com; Eydelberg, Alex <alex.eydelberg@intel.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
>