From mboxrd@z Thu Jan  1 00:00:00 1970
From: Caitlin Bestler <caitlin.bestler@gmail.com>
Subject: Re: [PATCH v3 3/6] vbus: add a "vbus-proxy" bus model for vbus_driver
	objects
Date: Thu, 20 Aug 2009 13:58:20 -0700
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Cc: Gregory Haskins <gregory.haskins@gmail.com>,
	Ingo Molnar <mingo@elte.hu>, kvm@vger.kernel.org,
	alacrityvm-devel@lists.sourceforge.net,
	linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Patrick Mullaney <pmullaney@novell.com>
To: Avi Kivity <avi@redhat.com>
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On Wed, Aug 19, 2009 at 1:37 PM, Avi Kivity<avi@redhat.com> wrote:

>
> Well I'm not an Infiniband expert. =C2=A0But from what I understand V=
MM bypass
> means avoiding the call to the VMM entirely by exposing hardware regi=
sters
> directly to the guest.
>

It enables clients to talk directly to the hardware. Whether or not
that involves
registers would be model specific. But frequently the queues being writ=
ten
were in the client's memory, and only a "doorbell ring" involved actual=
 device
resources.

But whatever the mechanism, it enables the client to provide buffer add=
resses
directly to the hardware in a manner that cannot damage another client.=
 The two
key requirements are a) client cannot enable access to pages that it do=
es
not already have access to, and b) client can delegate that authority t=
o the
Adapter without needing to invoke OS or Hypervisor on a per message
basis.


Traditionally that meant that memory maps ("Regions") were created on t=
he
privileged path to enable fast/non-privileged references by the client.