[PATCH 0/4] KVM: arm/arm64: vgic: Virtual interrupt grouping support

Christoffer Dall
2018-06-25T10:13:47Z

On Mon, Jun 25, 2018 at 10:52:41AM +0100, Peter Maydell wrote:
> On 24 June 2018 at 23:10, Christoffer Dall <christoffer.dall@arm.com> wrote:
> > This small series addresses a peculiarity of the current VGIC
> > implementation, namely that we don't support interrupt grouping.
> >
> > KVM either implements a GICv2 without support for the security
> > extensions, or a GICv3 with DS=1. For GICv2, on systems without the
> > security extensions, group 0 interrupts can be configured to be either
> > signalled as FIQs or as IRQs by the VM, whereas group 1 interrupts are
> > always IRQs. For GICv3, with DS=1, group 1 interrupts are always IRQs
> > and group 0 interrupts are always FIQs, and there is no concept of
> > secure vs. non-secure group 1 interrupts when DS=1.
> >
> > We were treating all interrupts on GICv2 as group 0, but yet telling the
> > geust that they were group 1. The first patch changes this behavior,
> > which seems to have no effect on no known guests, but still.
> >
> > The remaining patches introduce proper interrupt grouping support, along
> > with MMIO accessors for the VM and userspace to retrieve and set the
> > which group SGIs, PPIs, and SPIs belong to. LPIs are always group 1
> > interrupts as per the architecture, and there is no way to modify this
> > configuration (no IGROUPR registers for LPIs or equivalent ITS
> > commands).
>
> How do we handle migration compatibility here? For instance if
> we have a guest running on an old kernel with GICv2, presumably
> it reports the GICD_IGROUPRn to userspace as being all-1 (but
> they behave as group 0); then when we load on the new kernel
> the new kernel will honour the IGROUPRn values and the interrupts
> will start behaving like group 1 ? Maybe this works because the
> guest is expecting them to be signalled as IRQ either way...
>

Interesting. For GICv2, I think this will actually break, and the
migrated Linux guest won't see any interrupts anymore (Linux only sets
bit 0 in GICD_CTLR). For GICv3, everything stays group 1 unless the
guest is specifically trying to configure group 0 for something.

I suppose this means we need to let userspace request
ARCH_COMPLIANT_GICV2 and otherwise ignore userspace writes to IGROUPR?

Thanks,
-Christoffer