Re: [PATCH v4 09/10] KVM: arm/arm64: vgic: Let userspace opt-in to writable v2 IGROUPR

From: Andrew Jones <drjones@redhat.com>
To: Christoffer Dall <christoffer.dall@arm.com>
Cc: kvm@vger.kernel.org, Marc Zyngier <marc.zyngier@arm.com>,
	Andre Przywara <andre.przywara@arm.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 09/10] KVM: arm/arm64: vgic: Let userspace opt-in to writable v2 IGROUPR
Date: Thu, 19 Jul 2018 20:17:21 +0200	[thread overview]
Message-ID: <20180719181721.k4q2gmzr6wuy6nay@kamzik.brq.redhat.com> (raw)
In-Reply-To: <1531746387-7033-10-git-send-email-christoffer.dall@arm.com>

On Mon, Jul 16, 2018 at 03:06:26PM +0200, Christoffer Dall wrote:
> Simply letting IGROUPR be writable from userspace would break
> migration from old kernels to newer kernels, because old kernels
> incorrectly report interrupt groups as group 1.  This would not be a big
> problem if userspace wrote GICD_IIDR as read from the kernel, because we
> could detect the incompatibility and return an error to userspace.
> Unfortunately, this is not the case with current userspace
> implementations and simply letting IGROUPR be writable from userspace for
> an emulated GICv2 silently breaks migration and causes the destination
> VM to no longer run after migration.
> 
> We now encourage userspace to write the read and expected value of
> GICD_IIDR as the first part of a GIC register restore, and if we observe
> a write to GICD_IIDR we know that userspace has been updated and has had
> a chance to cope with older kernels (VGICv2 IIDR.Revision == 0)
> incorrectly reporting interrupts as group 1, and therefore we now allow
> groups to be user writable.
> 
> Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
> ---
>  include/kvm/arm_vgic.h           |  3 +++
>  virt/kvm/arm/vgic/vgic-mmio-v2.c | 16 +++++++++++++++-
>  2 files changed, 18 insertions(+), 1 deletion(-)
>

Reviewed-by: Andrew Jones <drjones@redhat.com>