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

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Message-ID: <20100602100404.GF964@8bytes.org>
Date:	Wed, 2 Jun 2010 12:04:04 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...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 12:49:28PM +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

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