linux-kernel - Re: [PATCH] vfio: Allow sub-ranges to be unmapped

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Date:	Tue, 24 May 2011 22:10:31 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	pugs@...e.org
Cc:	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	qemu-devel@...gnu.org
Subject: Re: [PATCH] vfio: Allow sub-ranges to be unmapped

On Tue, 2011-05-10 at 12:06 -0600, Alex Williamson wrote:
> We're currently very strict in requiring that DMA unmaps are done
> using the exact same regions as used for the original mapping.
> In a VM environment, we may want to change sub-areas of a previous
> mapping without tearing down the entire region.  This might also
> also us to support ballooning or memory hotplug in a guest making
> use of a vfio device.
> 
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
> 
> Tom,
> 
> I'm a little confused why we have the list of mapped areas attached
> to the listener.  If we want to support multiple devices sharing the same
> uiommu, don't we instead need to create a separate object hosting this
> list with reference counting so multiple vdevs can point to it?  I would
> expect that's where the locked_page accounting would be moved to as well.
> There also seems to be support that each vdev could have multiple
> listeners, each with their own list of dma mapped areas, but we can
> only have one udomain per vdev, which just further confuses me.  Can you
> hlp me out?  Thanks,
> 
> Alex
> 
> Note the change of npage to unsigned long below is just to avoid needing
> to cast it every time we do a << PAGE_SHIFT to avoid overflow.

Self NAK on this last change, it breaks locked page accounting since we
rely on a sign bit there.  I'll try just using a macro instead to avoid
excessive casting.  The follow-up uiommu sharing patch is dependent on
this, so I'll respin both.  Thanks,

Alex

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