From: Nadav Har'El <nyh@math.technion.ac.il>
To: Chegu Vinod <chegu_vinod@hp.com>
Cc: Gleb Natapov <gleb@redhat.com>, kvm@vger.kernel.org
Subject: Re: Networking performance on a KVM Host (with no guests)
Date: Fri, 20 Apr 2012 22:35:49 +0300 [thread overview]
Message-ID: <20120420193549.GA30585@fermat.math.technion.ac.il> (raw)
In-Reply-To: <4F916F96.8070100@hp.com>
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.
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.
--
Nadav Har'El | Friday, Apr 20 2012,
nyh@math.technion.ac.il |-----------------------------------------
Phone +972-523-790466, ICQ 13349191 |A bird in the hand is safer than one
http://nadav.harel.org.il |overhead.
next prev parent reply other threads:[~2012-04-20 19:35 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-19 3:53 Networking performance on a KVM Host (with no guests) Chegu Vinod
2012-04-19 5:43 ` Gleb Natapov
2012-04-19 6:42 ` Jean-Philippe Menil
2012-04-20 14:15 ` Chegu Vinod
2012-04-20 19:35 ` Nadav Har'El [this message]
2012-04-21 0:30 ` Chegu Vinod
2012-04-22 11:09 ` Nadav Har'El
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