linux-kernel - Re: [PATCH v3 0/3] virtio DMA API core stuff

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Date:	Fri, 30 Oct 2015 16:54:58 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Andy Lutomirski <luto@...capital.net>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Joerg Roedel <jroedel@...e.de>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Christoph Hellwig <hch@....de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	KVM <kvm@...r.kernel.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH v3 0/3] virtio DMA API core stuff

(Sorry, missed part of this before).

On Thu, 2015-10-29 at 11:01 +0200, Michael S. Tsirkin wrote:
> Isn't this specified by the hypervisor? I don't think this is a good
> way to do this: guest security should be up to guest.

And it is. When the guest sees an IOMMU, it can choose to use it, or
choose not to (or choose to put it in passthrough mode). But as Jörg
says, we don't have a way for an individual  device driver to *request*
passthrough mode or not yet; the choice is made by the core IOMMU code
(iommu=pt on the command line) — or by the platform simply stating that
a given device isn't *covered* by an IOMMU, if that is indeed the case.

In *no* circumstance is it sane for a device driver just to "opt out"
of using the correct DMA API function calls, and expect that to
*magically* cause the IOMMU to be bypassed.

> > Everyone seems to agree that x86's emulated Q35 thing
> > is just buggy right now and should be taught to use the existing ACPI
> > mechanism for enumerating passthrough devices.
> 
> I'm not sure what ACPI has to do with it.
> It's about a way for guest users to specify whether
> they want to bypass an IOMMU for a given device.

No, it absolutely isn't. You might want that — and see the discussion
about DMA_ATTR_IOMMU_BYPASS if you do. But that is *utterly* irrelevant
to *this* discussion, in which you seem to be advocating that the
virtio drivers should remain buggy by just unilaterally not using the
DMA API.

> By the way, a bunch of code is missing on the QEMU side
> to make this useful:
> 1. virtio ignores the iommu
> 2. vhost user ignores the iommu
> 3. dataplane ignores the iommu
> 4. vhost-net ignores the iommu
> 5. VFIO ignores the iommu

No, those things are not useful for fixing the virtio driver bug under
discussion here. All we need to do is make the virtio drivers correctly
use the DMA API. They should never have passed review and been accepted
into the Linux kernel without that.

All we need to do first is make sure that the bug we have in the
PowerPC IOMMU code (and potentially ARM and/or SPARC?) is fixed, and
that it doesn't attempt to use an IOMMU that doesn't exist. And ensure
that the virtualised IOMMU on qemu/x86 isn't lying and claiming that it
translates for the virtio devices when it doesn't.

There are other things we might want to do — like fixing the IOMMU that
qemu can emulate, and actually making it work with real assigned
devices (currently it's totally hosed because it doesn't handle that
case at all). And potentially making the virtualised IOMMU actually
*do* translation for virtio devices (as opposed to just admitting
correctly that it doesn't). But those aren't strictly relevant here,
yet.

It's not clear what specific uses of the IOMMU you had in mind in your
above list — could you elucidate?

-- 
dwmw2

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