Re: Booting from PV disk driver (Was: Re: [PATCH 10/13] KVM: Wire up hypercall handler ..)

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Arjan van de Ven wrote:
> On Sun, 2007-02-25 at 07:17 +0200, Avi Kivity wrote:
>   
>> Arjan van de Ven wrote:
>>     
>>>> The easiest thing I can think of is to have the PV disk driver show up 
>>>> as an actual PCI device and to use a PCI option rom to hijack the 
>>>> appropriate interrupt.
>>>>     
>>>>         
>>> yeah that's a good plan. Being a PCI device also helps with the OS
>>> userspace knowing which module to load; even if the device itself never
>>> gets accessed. 
>>>
>>>   
>>>       
>> A problem with pci, at least if you use the pci interrupts, is that your 
>> interrupt might be shared with another device.  I guess a solution is 
>> not to use pci interrupts, but then we need some mechanism to allocate 
>> interrupts.
>>     
>
> shared interrupts aren't a big deal in Linux. at all.
> In fact, sharing all PV interrupts to one number is a performance
> enhancement ;)
>   

If you share a PV interrupt with a non-PV interrupt, then for each PV 
interrupt you have to check whether the non-PV interrupt fired.  That 
involves at least one expensive vmexit.

I agree that sharing PV interrupts is not very expensive (though I don't 
see why you call it an optimization - if you share 100 interrupts on one 
line you need to check 100 interrupt sources every time the interrupt 
fires).

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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