Re: [PATCH 3/8] KVM: Handle device assignment to guests

[Avi Kivity <avi@qumranet.com>]

Ben-Ami Yassour wrote:
> On Thu, 2008-07-17 at 11:31 +0300, Avi Kivity wrote:
>   
>> Ben-Ami Yassour wrote:
>>     
>>>>>>> +
>>>>>>> +/* FIXME: Implement the OR logic needed to make shared interrupts
>>>>>>> on + * this line behave properly + */
>>>>>>>
>>>>>>>       
>>>>>>>           
>>>>>>>               
>>>>>> Isn't this a showstopper?  There is no easy way for a user to avoid
>>>>>> sharing, especially as we have only three pci irqs at present.
>>>>>>         
>>>>>>             
>>> What do you mean by only 3 (for passthrough we use whatever interrupt
>>> the host is using for that device)?
>>>
>>>   
>>>       
>> There are two issue:
>> - shared host interrupts
>> - shared guest interrupts
>>
>> For shared host interrupts, I don't think there's a cost-effective 
>> solution, especially as hosts are transitioning to MSI.  But the problem 
>> the comment describes is shared guest interrupts, where the assigned 
>> device's interrupt is shared with another device (assigned or virtual).
>>
>> And currently we only have three shareable interrupts: 5, 10, and 11.
>>
>> See qemu's pci_set_irq() for the logic used to share interrupts.
>>     
>
> I think we'll leave this issue until we fix the more basic comments. In
> any case, how can we make qemu configure the assigned device to share an
> interrupt with another device (assigned or virtual)?
>
>   

It's the guest's responsibility to assign irq lines, when running in 
acpi mode.  In no-acpi mode, the bios assigns lines according to an 
algorithm in bios/rombios32.c:pci_bios_init_device().

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