[PATCH v3 1/4] KVM: arm64: vgic: Allow userspace to set IIDR revision 1

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

In order to preserve guest compatibility across kernel upgrades, allow
userspace to select GICD_IIDR revision 1. This allows compatibility with
the original behaviour from before commit d53c2c29ae0d ("KVM: arm/arm64:
vgic: Allow configuration of interrupt groups") where interrupt groups
are not guest-configurable.

When revision 1 is selected:
 - GICv2: IGROUPR reads as zero (group 0), writes are ignored
 - GICv3: IGROUPR reads as all-ones (group 1), writes are ignored
 - v2_groups_user_writable is not set

This is implemented by checking the implementation revision in
vgic_mmio_write_group() and suppressing writes when the revision is
below 2. The read side needs no change since the per-IRQ group reset
values already match the expected behaviour.

For GICv2, commit 32f8777ed92d7 ("KVM: arm/arm64: vgic: Let userspace
opt-in to writable v2 IGROUPR") introduced a confusing model where
IGROUPR registers were not writable from userspace by default until the
IIDR was written — even if it was written to the *same* as its default
value (which, in fact, was the only thing that userspace *could* set it
to before commit a0e6ae45af17 fixed the IIDR write path). Furthermore,
even when the v2_groups_user_writable flag wasn't set, the *guest*
could still actually write to the registers... but userspace couldn't
save/restore them. That default behaviour for GICv2 remains unchanged;
it can be fixed in a future commit.

Fixes: d53c2c29ae0d ("KVM: arm/arm64: vgic: Allow configuration of interrupt groups")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/arm64/kvm/vgic/vgic-mmio-v2.c | 3 +++
 arch/arm64/kvm/vgic/vgic-mmio-v3.c | 4 ++++
 arch/arm64/kvm/vgic/vgic-mmio.c    | 4 ++++
 include/kvm/arm_vgic.h             | 1 +
 4 files changed, 12 insertions(+)

diff --git a/arch/arm64/kvm/vgic/vgic-mmio-v2.c b/arch/arm64/kvm/vgic/vgic-mmio-v2.c
index 0643e333db35..e5714f7fd2ec 100644
--- a/arch/arm64/kvm/vgic/vgic-mmio-v2.c
+++ b/arch/arm64/kvm/vgic/vgic-mmio-v2.c
@@ -20,6 +20,7 @@
  * Revision 1: Report GICv2 interrupts as group 0 instead of group 1
  * Revision 2: Interrupt groups are guest-configurable and signaled using
  * 	       their configured groups.
+ * Revision 3: GICv2 behaviour is unchanged from revision 2.
  */
 
 static unsigned long vgic_mmio_read_v2_misc(struct kvm_vcpu *vcpu,
@@ -96,6 +97,8 @@ static int vgic_mmio_uaccess_write_v2_misc(struct kvm_vcpu *vcpu,
 		case KVM_VGIC_IMP_REV_2:
 		case KVM_VGIC_IMP_REV_3:
 			vcpu->kvm->arch.vgic.v2_groups_user_writable = true;
+			fallthrough;
+		case KVM_VGIC_IMP_REV_1:
 			dist->implementation_rev = reg;
 			return 0;
 		default:
diff --git a/arch/arm64/kvm/vgic/vgic-mmio-v3.c b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
index 5913a20d8301..0130db71cfc9 100644
--- a/arch/arm64/kvm/vgic/vgic-mmio-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
@@ -74,8 +74,11 @@ bool vgic_supports_direct_sgis(struct kvm *kvm)
 /*
  * The Revision field in the IIDR have the following meanings:
  *
+ * Revision 1: Interrupt groups are not guest-configurable.
+ * 	       IGROUPR reads as all-ones (group 1), writes ignored.
  * Revision 2: Interrupt groups are guest-configurable and signaled using
  * 	       their configured groups.
+ * Revision 3: GICR_CTLR.{IR,CES} are advertised.
  */
 
 static unsigned long vgic_mmio_read_v3_misc(struct kvm_vcpu *vcpu,
@@ -196,6 +199,7 @@ static int vgic_mmio_uaccess_write_v3_misc(struct kvm_vcpu *vcpu,
 
 		reg = FIELD_GET(GICD_IIDR_REVISION_MASK, val);
 		switch (reg) {
+		case KVM_VGIC_IMP_REV_1:
 		case KVM_VGIC_IMP_REV_2:
 		case KVM_VGIC_IMP_REV_3:
 			dist->implementation_rev = reg;
diff --git a/arch/arm64/kvm/vgic/vgic-mmio.c b/arch/arm64/kvm/vgic/vgic-mmio.c
index 74d76dec9730..1b662744ec5b 100644
--- a/arch/arm64/kvm/vgic/vgic-mmio.c
+++ b/arch/arm64/kvm/vgic/vgic-mmio.c
@@ -73,6 +73,10 @@ void vgic_mmio_write_group(struct kvm_vcpu *vcpu, gpa_t addr,
 	int i;
 	unsigned long flags;
 
+	/* Revision 1 and below: groups are not guest-configurable. */
+	if (vgic_get_implementation_rev(vcpu) < KVM_VGIC_IMP_REV_2)
+		return;
+
 	for (i = 0; i < len * 8; i++) {
 		struct vgic_irq *irq = vgic_get_vcpu_irq(vcpu, intid + i);
 
diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
index 1388dc6028a9..16811ec03d54 100644
--- a/include/kvm/arm_vgic.h
+++ b/include/kvm/arm_vgic.h
@@ -372,6 +372,7 @@ struct vgic_dist {
 
 	/* Implementation revision as reported in the GICD_IIDR */
 	u32			implementation_rev;
+#define KVM_VGIC_IMP_REV_1	1 /* GICv2 interrupts as group 0 */
 #define KVM_VGIC_IMP_REV_2	2 /* GICv2 restorable groups */
 #define KVM_VGIC_IMP_REV_3	3 /* GICv3 GICR_CTLR.{IW,CES,RWP} */
 #define KVM_VGIC_IMP_REV_LATEST	KVM_VGIC_IMP_REV_3
-- 
2.51.0