From mboxrd@z Thu Jan  1 00:00:00 1970
From: Sheng Yang <sheng@linux.intel.com>
Subject: Re: [PATCH qemu-kvm] device assignment: default requires IOMMU
Date: Thu, 24 Dec 2009 14:56:00 +0800
Message-ID: <200912241456.00624.sheng@linux.intel.com>
References: <20091223224020.GB3305@sequoia.sous-sol.org> <1913B65B-382C-4838-8223-B48246E090F7@suse.de> <20091224065123.GD2814@verge.net.au>
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Cc: Alexander Graf <agraf@suse.de>, Chris Wright <chrisw@sous-sol.org>,
	Avi Kivity <avi@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	Dmitri Seletski <drjoms@gmail.com>
To: Simon Horman <horms@verge.net.au>
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On Thursday 24 December 2009 14:51:23 Simon Horman wrote:
> On Thu, Dec 24, 2009 at 01:45:34AM +0100, Alexander Graf wrote:
> > Am 23.12.2009 um 23:40 schrieb Chris Wright <chrisw@sous-sol.org>:
> > >[ resend, fixing email header, sorry for duplicate ]
> > >
> > >The default mode for device assignment is to rely on an IOMMU for
> > >proper translations and a functioning device in the guest.  The
> > >current
> > >logic makes this requirement advisory, and simply disables the request
> > >for IOMMU if one is not found on the host.  This makes for a confused
> > >user when the device assignment appears to work, but the device in the
> > >guest is not functioning  (I've seen about a half-dozen reports with
> > >this failure mode).
> > >
> > >Change the logic such that the default requires the IOMMU.  Period.
> > >If the host does not have an IOMMU, device assignment will fail.
> > >
> > >This is a user visible change, however I think the current
> > >situation is
> > >simply broken.
> > >
> > >And, of course, disabling the IOMMU requirement using the old:
> > >
> > >  -pcidevice host=[addr],dma=none
> > >
> > >or the newer:
> > >
> > >  -device pci-assign,host=[addr],iommu=0
> > >
> > >will do what it always did (not require an IOMMU, and fail to work
> > >properly).
> >
> > Yay!
> 
> Sounds good to me. Though I am curious to know the reasoning
> behind the current logic.
> 
Sounds pretty good. :)

I think maybe it due to we are interested in implementing PV DMA?

-- 
regards
Yang, Sheng