On Fri, Sep 16, 2016 at 02:25:01PM +0200, Alexander Graf wrote:
> 
> > On 16 Sep 2016, at 12:20, Marc Zyngier <marc.zyng...@arm.com> wrote:
> > 
> > Hi Alex,
> > 
> > On 16/09/16 07:26, Alexander Graf wrote:
> >> Some systems out there (well, one type in particular - the Raspberry Pi 
> >> series)
> >> do have virtualization capabilities in the core, but no ARM GIC interrupt
> >> controller.
> >> 
> >> To run on these systems, the cleanest route is to just handle all
> >> interrupt delivery in user space and only deal with IRQ pins in the core
> >> side in KVM.
> >> 
> >> This works pretty well already, but breaks when the guest starts to use
> >> architected timers, as these are handled straight inside kernel space 
> >> today.
> >> 
> >> This patch set allows user space to receive vtimer events as well as mask
> >> them, so that we can handle all vtimer related interrupt injection from 
> >> user
> >> space, enabling us to use architected timer with user space gic emulation.
> > 
> > I have already voiced my concerns in the past, including face to face,
> > and I'm going to repeat it: I not keen at all on adding a new userspace
> > interface that is going to bitrot extremely quickly.
> > 
> > Let's face it, this new ABI will have a single user, with a limited
> > shelf life. I understand that the RPi is a popular product, but it looks
> > fairly obvious that this kind of sub-standard HW will eventually
> > disappear. We'll then be left with a userspace ABI that will break at
> 
> I’m not 100% convinced that this is the case. Emulating the GIC in user space 
> can have other interesting use cases. For example, it might come in handy for 
> nesting. It may be useful for migrating a gicv2 VM to a gicv3 host without 
> gicv2 emulation as well.
> 

I don't see why you'd do this; the VGIC hardware can perfectly well be
used for nesting as well, and this works rather well.

> > every single release, given that nobody in the RPi community actually
> > uses a mainline kernel.
> 
> I actually verified all of this patch on 4.8-rc5 upstream, which is the only 
> 64bit kernel you can find for the RPi. So I’d expect the situation to change 
> with 64bit.
> 
> > And breaking this ABI will introduce userspace exploitable bugs, like
> > the one you've already shown. If anything, I would have loved to
> > completely kill the whole userspace GIC, because nobody cares. Yes, I
> > understand it is fun to have KVM running on the RPi. But the maintenance
> > costs far outweigh the fun aspect already.
> 
> Having CPU pins accessible is very useful for use cases of KVM that are 
> beyond your traditional VM.
> 
> > You could still run KVM with an external emulated timer (not the arch
> > timer). No need for a new ABI for that.
> 
> That’s what I thought too, but turns out that it’s not quite as simple as I 
> hoped ;).

Why not?  I thought a few people had this running recently.  What were
the issues?  Perhaps I can convince someone to submit the patches they
used if useful.

Thanks,
-Christoffer
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