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

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Date:	Wed, 02 Jun 2010 08:40:06 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Chris Wright <chrisw@...s-sol.org>
CC:	Tom Lyon <pugs@...n-about.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, joro@...tes.org,
	hjk@...utronix.de, gregkh@...e.de, aafabbri@...co.com,
	scofeldm@...co.com, alex.williamson@...hat.com
Subject: Re: [PATCH] VFIO driver: Non-privileged user level PCI drivers

On 06/02/2010 08:29 AM, Chris Wright wrote:
> * Avi Kivity (avi@...hat.com) wrote:
>    
>> On 06/02/2010 12:26 AM, Tom Lyon wrote:
>>      
>>> I'm not really opposed to multiple devices per domain, but let me point out how I
>>> ended up here.  First, the driver has two ways of mapping pages, one based on the
>>> iommu api and one based on the dma_map_sg api.  With the latter, the system
>>> already allocates a domain per device and there's no way to control it. This was
>>> presumably done to help isolation between drivers.  If there are multiple drivers
>>> in the user level, do we not want the same isoation to apply to them?
>>>        
>> In the case of kvm, we don't want isolation between devices, because
>> that doesn't happen on real hardware.
>>      
> Sure it does.  That's exactly what happens when there's an iommu
> involved with bare metal.
>    

But we are emulating a machine without an iommu.

When we emulate a machine with an iommu, then yes, we'll want to use as 
many domains as the guest does.

>> So if the guest programs
>> devices to dma to each other, we want that to succeed.
>>      
> And it will as long as ATS is enabled (this is a basic requirement
> for PCIe peer-to-peer traffic to succeed with an iommu involved on
> bare metal).
>
> That's how things currently are, i.e. we put all devices belonging to a
> single guest in the same domain.  However, it can be useful to put each
> device belonging to a guest in a unique domain.  Especially as qemu
> grows support for iommu emulation, and guest OSes begin to understand
> how to use a hw iommu.
>    

Right, we need to keep flexibility.

>>> And then there's the fact that it is possible to have multiple disjoint iommus on a system,
>>> so it may not even be possible to bring 2 devices under one domain.
>>>        
>> That's indeed a deficiency.
>>      
> Not sure it's a deficiency.  Typically to share page table mappings
> across multiple iommu's you just have to do update/invalidate to each
> hw iommu that is sharing the mapping.  Alternatively, you can use more
> memory and build/maintain identical mappings (as Tom alludes to below).
>    

Sharing the page tables is just an optimization, I was worried about 
devices in separate domains not talking to each other.  if ATS fixes 
that, great.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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