linux-kernel - Re: [RFC v2 06/10] KVM: arm/arm64: Update the physical timer interrupt level

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Date:   Mon, 30 Jan 2017 19:41:31 +0100
From:   Christoffer Dall <christoffer.dall@...aro.org>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     Jintack Lim <jintack@...columbia.edu>, pbonzini@...hat.com,
        rkrcmar@...hat.com, linux@...linux.org.uk, catalin.marinas@....com,
        will.deacon@....com, andre.przywara@....com, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 06/10] KVM: arm/arm64: Update the physical timer
 interrupt level

On Mon, Jan 30, 2017 at 05:50:03PM +0000, Marc Zyngier wrote:
> On 30/01/17 15:02, Christoffer Dall wrote:
> > On Sun, Jan 29, 2017 at 03:21:06PM +0000, Marc Zyngier wrote:
> >> On Fri, Jan 27 2017 at 01:04:56 AM, Jintack Lim <jintack@...columbia.edu> wrote:
> >>> Now that we maintain the EL1 physical timer register states of VMs,
> >>> update the physical timer interrupt level along with the virtual one.
> >>>
> >>> Note that the emulated EL1 physical timer is not mapped to any hardware
> >>> timer, so we call a proper vgic function.
> >>>
> >>> Signed-off-by: Jintack Lim <jintack@...columbia.edu>
> >>> ---
> >>>  virt/kvm/arm/arch_timer.c | 20 ++++++++++++++++++++
> >>>  1 file changed, 20 insertions(+)
> >>>
> >>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> >>> index 0f6e935..3b6bd50 100644
> >>> --- a/virt/kvm/arm/arch_timer.c
> >>> +++ b/virt/kvm/arm/arch_timer.c
> >>> @@ -180,6 +180,21 @@ static void kvm_timer_update_mapped_irq(struct kvm_vcpu *vcpu, bool new_level,
> >>>  	WARN_ON(ret);
> >>>  }
> >>>  
> >>> +static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
> >>> +				 struct arch_timer_context *timer)
> >>> +{
> >>> +	int ret;
> >>> +
> >>> +	BUG_ON(!vgic_initialized(vcpu->kvm));
> >>
> >> Although I've added my fair share of BUG_ON() in the code base, I've
> >> since reconsidered my position. If we get in a situation where the vgic
> >> is not initialized, maybe it would be better to just WARN_ON and return
> >> early rather than killing the whole box. Thoughts?
> >>
> > 
> > The distinction to me is whether this will cause fatal crashes or
> > exploits down the road if we're working on uninitialized data.  If all
> > that can happen if the vgic is not initialized, is that the guest
> > doesn't see interrupts, for example, then a WARN_ON is appropriate.
> > 
> > Which is the case here?
> > 
> > That being said, do we need this at all?  This is in the critial path
> > and is actually measurable (I know this from my work on the other timer
> > series), so it's better to get rid of it if we can.  Can we simply
> > convince ourselves this will never happen, and is the code ever likely
> > to change so that it gets called with the vgic disabled later?
> 
> That'd be the best course of action. I remember us reworking some of
> that in the now defunct vgic-less series. Maybe we could salvage that
> code, if only for the time we spent on it...
> 
Ah, we never merged it?  Were we waiting on a userspace implementation
or agreement on the ABI?

There was definitely a useful cleanup with the whole enabled flag thing
on the timer I remember.

Thanks,
-Christoffer