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

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Message-ID: <CALCETrVkE3=KCxK-_bf-P8UfTS96Vck+QPE8EsLvP7THxpLe1A@mail.gmail.com>
Date:	Tue, 10 Nov 2015 10:54:37 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Joerg Roedel <jroedel@...e.de>, KVM <kvm@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	sparclinux@...r.kernel.org
Subject: Re: [PATCH v4 0/6] virtio core DMA API conversion

On Nov 10, 2015 2:38 AM, "Benjamin Herrenschmidt"
<benh@...nel.crashing.org> wrote:
>
> 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.

Does that work on powerpc on existing kernels?

Anyway, here's another crazy idea: make the quirk assume that the
IOMMU is bypasses if and only if the weak barriers bit is set on
systems that are missing the new DT binding.

--Andy

>
> Cheers,
> Ben.
>
>
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