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

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Date:	Thu, 01 Jul 2010 09:29:04 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Tom Lyon <pugs@...co.com>
Cc:	randy.dunlap@...cle.com, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, chrisw@...s-sol.org, joro@...tes.org,
	hjk@...utronix.de, mst@...hat.com, 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 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

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