Re: PCI passthrough resource remapping

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Chris Wright wrote:
> * Alexander Graf (agraf@xxxxxxx) wrote:
>   
>> Kenni Lund wrote:
>>     
>>> 2010/3/31 Kenni Lund <kenni@xxxxxxx>:
>>>   
>>>       
>>>> Booting the guest and tuning to a DVB-T channel _works_ !!! :-D Thanks
>>>> a lot for your help...I have one more question, though: If I have two
>>>> devices (like the ivtv tuner and the USB card) and they share an IRQ,
>>>> if I then assign BOTH of them to the same guest, will it then work?
>>>>
>>>> Alexander, the patch works, I hope to see it in a stable release in
>>>> the near future ;)
>>>>     
>>>>         
>>> Unfortunately, I have a correction to this :( It _almost_ works. I had
>>> some video/audio artifacts which I though was caused by bad reception,
>>> but after switching the DVB-T tuner back and forth between the PCI USB
>>> card and a laptop, it got clear to me, that this was a passthrough
>>> issue. I get no errors in dmesg on the host or in the guest.
>>>
>>> I recorded a short videoclip which illustrates the issue:
>>> https://docs.google.com/leaf?id=0B-_nZameGeN-NDk4YzQ3N2EtMmEzMi00NTU4LWFjMjgtNzkxMzcxYzg2MTM1&hl=en
>>>   
>>>       
>> Hrm, I'm not sure these would be related to the small BAR region patch.
>> It looks more like a timing issue.
>>     
>
> small BAR == slow path == timing issue?
>   

Could be, yeah. The only chance I see to avoid that is to have in-kernel
small BAR code that would issue the MMIO accesses without the userspace
churn. One thing I could imagine would be < PAGE_SIZE memory slots. Then
userspace just maps the 4K to the BAR, but passes only a part of it on
as slot.


Alex

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