Re: [Qemu-devel] [PATCH 4/5] hw: arm_gic: Support setting/getting binary point reg

[Christoffer Dall <christoffer.dall@linaro.org>]

On Sat, Sep 14, 2013 at 10:46:37AM +0100, Peter Maydell wrote:
> On 14 September 2013 02:52, Christoffer Dall
> <christoffer.dall@linaro.org> wrote:
> > On Fri, Sep 06, 2013 at 03:41:04PM +0100, Peter Maydell wrote:
> >> The TCG QEMU GIC model is currently adopting the
> >> "GIC without Security Extensions" model, which implies
> >> that we should be implementing GIC_ABPR too. What
> >> model does KVM's in-kernel vGIC use? (ie what state
> >> does it put into binary_point[0] and [1] on save/load)?
> >>
> > We put whatever the guest writes into the GICV_BPR and GICV_ABPR, but
> > the in-kernel distributor does not care about priorities at all and
> > considers all interrupts to be group 0 interrupts, which just happens to
> > work for Linux.
> >
> > So yes, we should implement the GIC_ABPR, but there's no need for it
> > yet.  But, if I don't add these fields the guest will (by reading the
> > GICC_[A]BPR registers) be able to tell it was migrated, but it will not
> > influence anything on the distributor level.  Therefore, by adding these
> > fields we support the kernel's limited model fully without adding a
> > bunch of code that we can't really test in real life anyhow, and it
> > doesn't prevent us from adding full grouping support later on.
> 
> I agree we should have the fields for KVM's benefit. I think we
> should also add simple reads-as-written code in the TCG
> side so they're both accessible. State that the TCG implementation
> can't access is a bit weird. We should probably also call
> the fields bpr[NCPU] and abpr[NCPU] or something, so they're
> a little more self-documenting about what they represent.
> 
I already added the accessors for the TCG side?

I will rename to bpr and abpr.

-Christoffer