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

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Date:	Tue, 10 Nov 2015 15:44:10 -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 Tue, Nov 10, 2015 at 2:27 PM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
> On Tue, 2015-11-10 at 10:54 -0800, Andy Lutomirski wrote:
>>
>> 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.
>
> "New DT bindings" doesn't mean much ... how do we change DT bindings on
> existing machines with a FW in flash ?
>
> What about partition <-> partition virtio such as what we could do on
> PAPR systems. That would have the weak barrier bit.
>

Is it partition <-> partition, bypassing IOMMU?

I think I'd settle for just something that doesn't regress
non-experimental setups that actually work today and that allow new
setups (x86 with fixed QEMU and maybe something more complicated on
powerpc and/or sparc) to work in all cases.

We could certainly just make powerpc and sparc continue bypassing the
IOMMU until someone comes up with a way to fix it.  I'll send out some
patches that do that, and maybe that'll help this make progress.

--Andy
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