Re: [PATCH 0/3] KVM: arm: Implement software vGICv2 emulation

[Andre Przywara <andre.przywara@arm.com>]

Hi,

On 29/06/15 13:52, Christoffer Dall wrote:
> Hi Pavel,
> 
> [Please cc the kvm/arm list for such patches according to the
> MAINTAINERS file in the future]
> 
> On Mon, Jun 29, 2015 at 12:53:46PM +0300, Pavel Fedin wrote:
>> Some hardware (like Raspberry Pi 2) is capable of running KVM, however lacks
>> functional vGIC registers. This series introduces software vGIC emulation for
>> such machines, allowing to fully use virtualization capabilities
> 
> Is this rather esoteric use case really worth the extra code in the
> kernel? 

I wonder if these patches would pave the way to support running GICv2
guests on GICv3s without compat support? Admittedly not a really
compelling use case either, but at least worth discussing, I think.

Also if this will make the hack needed to enable KVM on RPi2 smaller,
I'd rather embrace this one than letting any random hacks appear on that
RPi kernel tree (patches which I have seen already on some other repo).
If I get this correctly, there are some efforts currently to get closer
to mainline with the RPi tree.

Pavel, is this "broken" GIC you are talking about going to appear in a
publicly available SoC? If yes, you could either state this right now or
send it later once you can talk publicly.

Marc, Christoffer:
So is this GICv2 CPU interface emulation totally out of question for us
or is it worth at least commenting on the patches?

Cheers,
Andre.

> I really feel that pure emulation should happen in userspace
> unless there's a very good reason for doing it in the kernel, such as a
> clearly measureable difference in performance, etc.
> 
> I would much rather see a version of this where a userspace provided GIC
> works with the in-kernel arch timers support.
> 
> -Christoffer
>