linux-kernel - Re: [PATCH V2] VFIO driver: Non-privileged user level PCI drivers

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Message-ID: <20100701153115.GA2008@redhat.com>
Date:	Thu, 1 Jul 2010 18:31:15 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Tom Lyon <pugs@...co.com>, randy.dunlap@...cle.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	chrisw@...s-sol.org, joro@...tes.org, hjk@...utronix.de,
	avi@...hat.com, gregkh@...e.de, aafabbri@...co.com,
	scofeldm@...co.com, Donald Dutile <ddutile@...hat.com>
Subject: Re: [PATCH V2] VFIO driver: Non-privileged user level PCI drivers

On Thu, Jul 01, 2010 at 09:29:04AM -0600, Alex Williamson wrote:
> On Tue, 2010-06-08 at 14:21 -0700, Tom Lyon wrote:
> > +The VFIO_DMA_MASK ioctl is used to set the maximum permissible DMA address
> > +(device dependent). It takes a single unsigned 64 bit integer as an argument.
> > +This call also has the side effect of enabling PCI bus mastership.
> 
> Hi Tom,
> 
> This interface doesn't make sense for the MAP_IOVA user.  Especially in
> qemu, we have no idea what the DMA mask is for the device we're
> assigning.  It doesn't really matter though because the guest will use
> bounce buffers internally once it loads the device specific drivers and
> discovers the DMA mask.  This only seems relevant if we're using a
> DMA_MAP call that gets to pick the dmaaddr, so I'd propose we only make
> this a required call for that interface, and create a separate ioctl for
> actually enabling bus master.  Thanks,
> 
> Alex

I expect there's no need for a separate ioctl to do this:
you can do this by write to the control register.

-- 
MST
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