hw/intc/arm_gicv3: fix handling of LPIs in list registerspull-target-arm-20211129

It is valid for an OS to put virtual interrupt ID values into the
list registers ICH_LR<n> which are greater than 1023.  This
corresponds to (for example) KVM using the in-kernel emulated ITS to
give a (nested) guest an ITS.  LPIs are delivered by the L1 kernel to
the L2 guest via the list registers in the same way as non-LPI
interrupts.

QEMU's code for handling writes to ICV_IARn (which happen when the L2
guest acknowledges an interrupt) and to ICV_EOIRn (which happen at
the end of the interrupt) did not consider LPIs, so it would
incorrectly treat interrupt IDs above 1023 as invalid.  Fix this by
using the correct condition, which is gicv3_intid_is_special().

Note that the condition in icv_dir_write() is correct -- LPIs
are not valid there and so we want to ignore both "special" ID
values and LPIs.

(In the pseudocode this logic is in:
 - VirtualReadIAR0(), VirtualReadIAR1(), which call IsSpecial()
 - VirtualWriteEOIR0(), VirtualWriteEOIR1(), which call
     VirtualIdentifierValid(data, TRUE) meaning "LPIs OK"
 - VirtualWriteDIR(), which calls VirtualIdentifierValid(data, FALSE)
     meaning "LPIs not OK")

This bug doesn't seem to have any visible effect on Linux L2 guests
most of the time, because the two bugs cancel each other out: we
neither mark the interrupt active nor deactivate it.  However it does
mean that the L2 vCPU priority while the LPI handler is running will
not be correct, so the interrupt handler could be unexpectedly
interrupted by a different interrupt.

(NB: this has nothing to do with using QEMU's emulated ITS.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>

1 files changed, 2 insertions, 3 deletions

diff --git a/hw/intc/arm_gicv3_cpuif.c b/hw/intc/arm_gicv3_cpuif.c
index 7fbc36ff41..7fba931450 100644
--- a/hw/intc/arm_gicv3_cpuif.c
+++ b/hw/intc/arm_gicv3_cpuif.c
@@ -653,7 +653,7 @@ static uint64_t icv_iar_read(CPUARMState *env, const ARMCPRegInfo *ri)
 
         if (thisgrp == grp && icv_hppi_can_preempt(cs, lr)) {
             intid = ich_lr_vintid(lr);
-            if (intid < INTID_SECURE) {
+            if (!gicv3_intid_is_special(intid)) {
                 icv_activate_irq(cs, idx, grp);
             } else {
                 /* Interrupt goes from Pending to Invalid */
@@ -1265,8 +1265,7 @@ static void icv_eoir_write(CPUARMState *env, const ARMCPRegInfo *ri,
     trace_gicv3_icv_eoir_write(ri->crm == 8 ? 0 : 1,
                                gicv3_redist_affid(cs), value);
 
-    if (irq >= GICV3_MAXIRQ) {
-        /* Also catches special interrupt numbers and LPIs */
+    if (gicv3_intid_is_special(irq)) {
         return;
     }