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

["Kay, Allen M" <allen.m.kay@xxxxxxxxx>]

Base on my understanding of the Neocleus' passthrough patch, it seems
all devices sharing that interrupt will get the double number of
interrupts.  This means if a interrupt is shared between a NIC device
used by a HVM guest and a SATA device used by dom0, the SATA driver in
dom0 will also get twice the number of interrupts.  Am I correct?

Allen 

>-----Original Message-----
>From: Guy Zana [mailto:guy@xxxxxxxxxxxx] 
>Sent: Wednesday, May 30, 2007 11:05 PM
>To: Keir Fraser; Kay, Allen M; xen-devel@xxxxxxxxxxxxxxxxxxx
>Subject: RE: [Xen-devel] [VTD][patch 0/5] HVM device 
>assignment using vt-d
>
>
>> -----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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