Re: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

[Raj, Ashok]

On Sun, Nov 08, 2020 at 06:34:55PM +0000, David Woodhouse wrote:
> > 
> > When we do interrupt remapping support in guest which would be required 
> > if we support x2apic in guest, I think this is something we should look into more 
> > carefully to make this work.
> 
> No, interrupt remapping is not required for X2APIC in guests
> 
> They can have X2APIC and up to 32768 CPUs without needing interrupt

How is this made available today without interrupt remapping? 

I thought without IR, the destination ID is still limited to only 8 bits?

On native, even if you have less than 255 cpu's but the APICID are sparsly 
distributed due to platform rules, the x2apic id could be more than 8 bits. 
Which is why the spec requires IR when x2apic is enabled.

> remapping at all. Only if they want more than 32768 vCPUs, or to do
> nested virtualisation and actually remap for the benefit of *their*
> (L2+) guests would they need IR.