Re: [PATCH v4 0/6] virtio core DMA API conversion

From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Joerg Roedel <jroedel@suse.de>, KVM <kvm@vger.kernel.org>,
	linux-s390 <linux-s390@vger.kernel.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Sebastian Ott <sebott@linux.vnet.ibm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Christoph Hellwig <hch@lst.de>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Andy Lutomirski <luto@kernel.org>,
	sparclinux@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Linux Virtualization <virtualization@lists.linux-foundation.org>,
	David Woodhouse <dwmw2@infradead.org>,
	"David S. Miller" <davem@davemloft.net>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [PATCH v4 0/6] virtio core DMA API conversion
Date: Tue, 10 Nov 2015 21:37:54 +1100	[thread overview]
Message-ID: <1447151874.31884.82.camel@kernel.crashing.org> (raw)
In-Reply-To: <CALCETrVPQc04Ah7FXcRMb4RhpUJ1WPCoC_4dbacB8a+u5XpmwA@mail.gmail.com>

On Mon, 2015-11-09 at 21:35 -0800, Andy Lutomirski wrote:
> 
> We could do it the other way around: on powerpc, if a PCI device is in
> that range and doesn't have the "bypass" property at all, then it's
> assumed to bypass the IOMMU.  This means that everything that
> currently works continues working.  If someone builds a physical
> virtio device or uses another system in PCIe target mode speaking
> virtio, then it won't work until they upgrade their firmware to set
> bypass=0.  Meanwhile everyone using hypothetical new QEMU also gets
> bypass=0 and no ambiguity.
>
> vfio will presumably notice the bypass and correctly refuse to map any
> current virtio devices.
> 
> Would that work?

That would be extremely strange from a platform perspective. Any device
in that vendor/device range would bypass the iommu unless some new
property "actually-works-like-a-real-pci-device" happens to exist in
the device-tree, which we would then need to define somewhere and
handle accross at least 3 different platforms who get their device-tree 
from widly different places.

Also if tomorrow I create a PCI device that implements virtio-net and
put it in a machine running IBM proprietary firmware (or Apple's or
Sun's), it won't have that property...

This is not hypothetical. People are using virtio to do point-to-point
communication between machines via PCIe today.

Cheers,
Ben.

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