Date: Thu, 1 Jul 2010 18:31:15 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: Tom Lyon <pugs@cisco.com>, randy.dunlap@oracle.com,
       linux-kernel@vger.kernel.org, kvm@vger.kernel.org, chrisw@sous-sol.org,
       joro@8bytes.org, hjk@linutronix.de, avi@redhat.com, gregkh@suse.de,
       aafabbri@cisco.com, scofeldm@cisco.com,
       Donald Dutile <ddutile@redhat.com>
Subject: Re: [PATCH V2] VFIO driver: Non-privileged user level PCI drivers
Message-ID: <20100701153115.GA2008@redhat.com>
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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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