From mboxrd@z Thu Jan  1 00:00:00 1970
From: Chegu Vinod <chegu_vinod@hp.com>
Subject: Re: Networking performance on a KVM Host (with no guests)
Date: Sat, 21 Apr 2012 00:30:34 +0000 (UTC)
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References: <loom.20120419T053941-656@post.gmane.org> <20120419054306.GV11918@redhat.com> <4F916F96.8070100@hp.com> <20120420193549.GA30585@fermat.math.technion.ac.il>
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Nadav Har'El <nyh <at> math.technion.ac.il> writes:

> 
> On Fri, Apr 20, 2012, Chegu Vinod wrote about "Re: Networking performance on a 
KVM Host (with no guests)":
> > Removing the "intel_iommu=on" boot time parameter  in the Config 1
> > case seemed to help
> 
> "intel_iommu=on" is essential with you're mostly running guests *and*
> using device assignment.
> 

Yes. I do understand that...

> However, unfortunately, it also has a serious performance penalty for
> I/O in *host* processes:
> 
> When intel_iommu=on, Linux (completely unrelated to KVM) adds a new level
> of protection which didn't exist without an IOMMU - the network card, which
> without an IOMMU could write (via DMA) to any memory location, now is
> not allowed - the card can only write to memory locates which the OS
> wanted it to write. Theoretically, this can protect the OS against
> various kinds of attacks. But what happens now is that every time that
> Linux passes a new buffer to the card, it needs to change the IOMMU
> mappings. This noticably slows down I/O, unfortunately.
> 

Hmm...  So if one were to have a private link setup between two hosts to drive 
some traffic through between the hosts... then we can't expect to get line rate 
on that private NIC. 

Thanks much for your response !
Vinod