linux-kernel - Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api

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Date:	Wed, 27 Apr 2016 18:09:31 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	David Woodhouse <dwmw2@...radead.org>,
	Kevin Wolf <kwolf@...hat.com>, Wei Liu <wei.liu2@...rix.com>,
	Andy Lutomirski <luto@...nel.org>, qemu-block@...gnu.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Jason Wang <jasowang@...hat.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"qemu-devel@...gnu.org Developers" <qemu-devel@...gnu.org>,
	peterx@...hat.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Amit Shah <amit.shah@...hat.com>,
	iommu@...ts.linux-foundation.org,
	Stefan Hajnoczi <stefanha@...hat.com>,
	kvm list <kvm@...r.kernel.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Anthony PERARD <anthony.perard@...rix.com>
Subject: Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api

On Wed, Apr 27, 2016 at 04:58:51PM +0200, Joerg Roedel wrote:
> On Wed, Apr 27, 2016 at 05:54:57PM +0300, Michael S. Tsirkin wrote:
> > Point is, QEMU is not the only virtio implementation out there.
> > So we can't know no virtio implementations have an IOMMU as long as
> > linux supports this IOMMU.
> > virtio always used physical addresses since it was born and if it
> > changes that it must do this in a way that does not break existing
> > users.
> 
> FWIW, virtio in qemu can continue to just use physical addresses. But
> qemu needs to advertise that fact correctly to the OS in the DMAR table.
> This way old kernels (where virtio does not use DMA-API) will also
> continue to work on the fixed qemu.
> 
> 
> 
> 	Joerg

It's not clear it can do this since DMAR tables seem to assume
a given slot is either bypassing IOMMU or going through it.
QEMU allows reusing same slot for virtio and non-virtio devices.

Besides, this patch is not about it, it's a flag for QEMU to
tell guest that it can trust DMAR tables.

-- 
MST