linux-kernel - Re: [PATCH 1/4] iommu: Add iommu_device_group callback and iommu

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Date:	Thu, 01 Dec 2011 10:33:30 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	David Gibson <dwg@....ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	joerg.roedel@....com, iommu@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, chrisw@...hat.com, agraf@...e.de,
	scottwood@...escale.com, B08248@...escale.com
Subject: Re: [PATCH 1/4] iommu: Add iommu_device_group callback and
 iommu_group sysfs entry

On Wed, 2011-11-30 at 23:48 -0700, Alex Williamson wrote:
> I'm not sure if we're getting into VM usage with "assigning" terminology
> above.  You're free to architect qemu however you want on POWER to make
> groups be the assignable unit to a guest.  On x86, an individual device
> is the assignable unit for a guest.  Unassigned group devices will still
> be required to be held by vfio, they'll just be unused.  Thanks,

I'm not sure I understand this.

BY DEFINITION, the smallest assignable unit is the group, surely? Isn't
that the *point* of the groups? That the IOMMU cannot tell the
difference between the devices in the group?

So in *practice*, surely you cannot assign just *one* device from a
group? You can assign the while group, or nothing.

You might *pretend* to assign single devices, and we might try to cope
with the weirdness that happens when you want *one* device of a group to
be owned by one VM, another device in the same group to be owned by a
second VM, and a third device from the same group to be driven by a
native driver in the host. But why not just assign groups as a whole?
Surely that makes most sense?

Btw, did we get a quirk for the Ricoh multi-function devices which all
need to be in the same group because they do all their DMA from function
zero? I think we need another similar quirk for a Marvell SATA
controller which seems to do its AHCI DMA from its IDE function; see
https://bugzilla.redhat.com/757166

-- 
dwmw2

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