RE: [Xen-devel] [VTD][patch 0/5] HVM device assignment using vt-d

["Guy Zana" <guy@xxxxxxxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Keir Fraser
> Sent: Wednesday, May 30, 2007 10:56 PM
> To: Kay, Allen M; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [VTD][patch 0/5] HVM device 
> assignment using vt-d
> 

> 
> Actually I also know there are some other patches coming down 
> the pipeline to do pci passthrough to HVM guests without need 
> for hardware support (of course it is not so general; in 
> particular it will only work for one special hvm guest). 
> However, they deal with this interrupt issue quite cunningly, 
> by inverting the interrupt polarity so that they get 
> interrupts on both +ve and -ve edges of the INTx line. This 
> allows the virtual interrupt wire to be 'wiggled' precisely 
> according to the behaviour of the physical interrupt wire. 
> Which is rather nice, although of course it does double the 
> interrupt rate, which is not so great but perhaps acceptable 
> for the kind of low interrupt rate devices that most people 
> would want to hand off to a hvm guest.
> 

Just FYI.

Neocleus' pass-through patches performs the "change polarity" trick.
With changing the polarity, our motivation was to reflect the allocated 
device's assertion state to the HVM AS IS.

Regarding the performance, using a USB 2.0 storage device (working with DMA), a 
huge file copy was compared when working in pass-through, and when working in 
native (on the same OS), the time differences were negligible so I'm not sure 
yet about the impact of doubling the number of interrupts. The advantage of 
changing the polarity is the simplicity.

Anyways, We'll release some patches during the day so you could give your 
comments.

Thanks,
Guy.

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