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

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Date:	Wed, 2 Jun 2010 13:09:58 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Avi Kivity <avi@...hat.com>, Tom Lyon <pugs@...co.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	chrisw@...s-sol.org, hjk@...utronix.de, gregkh@...e.de,
	aafabbri@...co.com, scofeldm@...co.com
Subject: Re: [PATCH] VFIO driver: Non-privileged user level PCI drivers

On Wed, Jun 02, 2010 at 12:04:04PM +0200, Joerg Roedel wrote:
> On Wed, Jun 02, 2010 at 12:49:28PM +0300, Avi Kivity wrote:
> > On 06/02/2010 12:45 PM, Joerg Roedel wrote:
> >> IOMMU mapped memory can not be swapped out because we can't do demand
> >> paging on io-page-faults with current devices. We have to pin _all_
> >> userspace memory that is mapped into an IOMMU domain.
> >
> > vhost doesn't pin memory.
> >
> > What I proposed is to describe the memory map using an object (fd), and  
> > pass it around to clients that use it: kvm, vhost, vfio.  That way you  
> > maintain the memory map in a central location and broadcast changes to  
> > clients.  Only a vfio client would result in memory being pinned.
> 
> Ah ok, so its only about the database which keeps the mapping
> information.
> 
> > It can still work, but the interface needs to be extended to include  
> > dirty bitmap logging.
> 
> Thats hard to do. I am not sure about VT-d but the AMD IOMMU has no
> dirty-bits in the page-table. And without demand-paging we can't really
> tell what pages a device has written to. The only choice is to mark all
> IOMMU-mapped pages dirty as long as they are mapped.
> 
> 	Joerg

Or mark them dirty when they are unmapped.

-- 
MST
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