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

[Sun, Ning]

>From tboot perspective, it is ok to add the option "tboot_noforce" to Linux kernel Intel_iommu parameter for those performance hungry tboot users, so long as the users are aware of the security implication behind of this option.
 
Thanks,
-ning

-----Original Message-----
From: Shaohua Li [mailto:shli@xxxxxx] 
Sent: Sunday, April 09, 2017 9:31 PM
To: Sun, Ning <ning.sun@xxxxxxxxx>
Cc: Joerg Roedel <jroedel@xxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Wei, Gang <gang.wei@xxxxxxxxx>; hpa@xxxxxxxxxxxxxxx; mingo@xxxxxxxxxx; kernel-team@xxxxxx; srihan@xxxxxx; Eydelberg, Alex <alex.eydelberg@xxxxxxxxx>
Subject: Re: [RFC] x86/tboot: add an option to disable iommu force on

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@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
>