linux-kernel - Re: [PATCH v3 0/3] virtio DMA API core stuff

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Message-ID: <20151028161424-mutt-send-email-mst@redhat.com>
Date:	Wed, 28 Oct 2015 16:22:26 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Andy Lutomirski <luto@...nel.org>,
	linux-kernel@...r.kernel.org, Joerg Roedel <jroedel@...e.de>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Christoph Hellwig <hch@....de>, benh@...nel.crashing.org,
	KVM <kvm@...r.kernel.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-s390 <linux-s390@...r.kernel.org>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v3 0/3] virtio DMA API core stuff

On Wed, Oct 28, 2015 at 11:13:29PM +0900, David Woodhouse wrote:
> On Wed, 2015-10-28 at 16:05 +0200, Michael S. Tsirkin wrote:
> > 
> > Short answer - platforms need a way to discover, and express different
> > security requirements of different devices.
> 
> Sure. PLATFORMS need that. Do not let it go anywhere near your device
> drivers. Including the virtio drivers.

But would there be any users of this outside the virtio subsystem?
If no, maybe virtio core is a logical place to keep this.

> > If they continue to lack that, we'll need a custom API in virtio,
> > and while this seems a bit less elegant, I would not see that as
> > the end of the world at all, there are not that many virtio drivers.
> 
> No. If they continue to lack that, we fix them. This is a *platform*
> issue. The DMA API shall do the right thing. Do not second-guess it.
> 
> 
>  (From the other mail)

I don't have a problem with extending DMA API to address
more usecases.

> > > > OK so I guess that means we should prefer a transport-specific
> > > > interface in virtio-pci then.
> > >
> > > Why?
> > 
> > Because you said you are doing something device tree specific for 
> > ARM, aren't you?
> 
> Nonono. The ARM platform code might do that, and the DMA API on ARM
> *might* give you I/O virtual addresses that look a lot like the
> physical addresses you asked it to map. That's none of your business.
> Drivers use DMA API. No more talky.

Well for virtio they don't ATM. And 1:1 mapping makes perfect sense
for the wast majority of users, so I can't switch them over
until the DMA API actually addresses all existing usecases.


> -- 
> dwmw2
> 
> 


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