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

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Date:	Wed, 11 Nov 2015 11:11:14 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Joerg Roedel <jroedel@...e.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	David Woodhouse <dwmw2@...radead.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	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

On Sat, Oct 31, 2015 at 12:16:12AM +0900, Joerg Roedel wrote:
> On Thu, Oct 29, 2015 at 11:01:41AM +0200, Michael S. Tsirkin wrote:
> > Example: you have a mix of assigned devices and virtio devices. You
> > don't trust your assigned device vendor not to corrupt your memory so
> > you want to limit the damage your assigned device can do to your guest,
> > so you use an IOMMU for that.  Thus existing iommu=pt within guest is out.
> > 
> > But you trust your hypervisor (you have no choice anyway),
> > and you don't want the overhead of tweaking IOMMU
> > on data path for virtio. Thus iommu=on is out too.
> 
> IOMMUs on x86 usually come with an ACPI table that describes which
> IOMMUs are in the system and which devices they translate. So you can
> easily describe all devices there that are not behind an IOMMU.
> 
> The ACPI table is built by the BIOS, and the platform intialization code
> sets the device dma_ops accordingly. If the BIOS provides wrong
> information in the ACPI table this is a platform bug.

It doesn't look like I managed to put the point across.
My point is that IOMMU is required to do things like
userspace drivers, what we need is a way to express
"there is an IOMMU but it is part of device itself, use passthrough
 unless your driver is untrusted".

> > 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.
> 
> We have no way yet to request passthrough-mode per-device from the IOMMU
> drivers, but that can easily be added. But as I see it:
> 
> > 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
> 
> Qemu does not implement IOMMU translation for virtio devices anyway
> (which is fine), so it just should tell the guest so in the ACPI table
> built to describe the emulated IOMMU.
> 
> 
> 	Joerg

This is a short term limitation.


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