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

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Date:	Mon, 9 May 2016 13:09:24 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	David Woodhouse <dwmw2@...radead.org>
Cc:	Joerg Roedel <joro@...tes.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, peterx@...hat.com,
	linux-kernel@...r.kernel.org, Amit Shah <amit.shah@...hat.com>,
	iommu@...ts.linux-foundation.org,
	Stefan Hajnoczi <stefanha@...hat.com>, kvm@...r.kernel.org,
	cornelia.huck@...ibm.com,
	virtualization@...ts.linux-foundation.org,
	Anthony PERARD <anthony.perard@...rix.com>
Subject: Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api



On 28/04/2016 17:37, Michael S. Tsirkin wrote:
> > All the internally-emulated devices *can* be either translated or
> > untranslated. That's just a matter of software. Surely, you currently
> > *can't* have translated assigned devices (until someone implements the
> > whole VT-d page table shadowing or whatever), so you'll be barred from
> > assigning a device to a slot which *previously* had an untranslated
> > device. But so what? Put it in a different slot instead.
> 
> Unfortunately people got used to be able to put any device
> in any slot, and built external tools around that ability.
> It's rather painful to break this assumption.

Once you move to PCIe, a lot of things become more complicated.  This is
just one of them; instead of needing half a dozen PCI bridges, you'll
need half a dozen plus one.

Paolo