[PATCH v6 2/6] xen/arm: vgic-v2: Handle correctly byte write in ITARGETSR

[Julien Grall <julien.grall@citrix.com>]

During a store, the byte is always in the low part of the register (i.e
[0:7]).

We are incorrectly masking the register by using a shift of the byte
offset in the ITARGETSR while the byte is alwasy in r[0:7]. This will
result in a target list equal to 0 which is ignored by the emulation.

Because of that the guest will only be able to modify the first byte in
each ITARGETSR.

Furthermore, the body of the loop is retrieving the old target list
using the index of the byte.

To avoid modifying too much the loop, shift the byte stored to the correct
offset.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

----
    This change used to be embedded in "xen/arm: vgic: Optimize the way
    to store the target vCPU in the rank". It has been moved out to
    avoid having too much functional changes in a single patch.

    This patch is a good candidate to backport to Xen 4.6 and Xen 4.5.
    Without it a guest won't be able migrate an IRQ from one vCPU to
    another if it's using byte access to write in ITARGETSR.

    Note that if we backport this patch alone, the resulting code in
    earlier version of Xen will be complex to read. As the last patch
    of this serie should also be backported, I'm planning to request
    backport for the whole series.

    Changes in v6:
        - Add Stefano's acked-by

    Changes in v5:
        - Update commit message based on Ian's suggestion

    Changes in v4:
        - Patch added
---
 xen/arch/arm/vgic-v2.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/xen/arch/arm/vgic-v2.c b/xen/arch/arm/vgic-v2.c
index 041291c..486e497 100644
--- a/xen/arch/arm/vgic-v2.c
+++ b/xen/arch/arm/vgic-v2.c
@@ -353,11 +353,11 @@ static int vgic_v2_distr_mmio_write(struct vcpu *v, mmio_info_t *info,
         /* 8-bit vcpu mask for this domain */
         BUG_ON(v->domain->max_vcpus > 8);
         target = (1 << v->domain->max_vcpus) - 1;
-        if ( dabt.size == 2 )
-            target = target | (target << 8) | (target << 16) | (target << 24);
+        target = target | (target << 8) | (target << 16) | (target << 24);
+        if ( dabt.size == DABT_WORD )
+            target &= r;
         else
-            target = (target << (8 * (gicd_reg & 0x3)));
-        target &= r;
+            target &= (r << (8 * (gicd_reg & 0x3)));
         /* ignore zero writes */
         if ( !target )
             goto write_ignore;
@@ -381,7 +381,7 @@ static int vgic_v2_distr_mmio_write(struct vcpu *v, mmio_info_t *info,
 
             if ( new_target != old_target )
             {
-                irq = gicd_reg - GICD_ITARGETSR + (i / 8);
+                irq = (gicd_reg & ~0x3) - GICD_ITARGETSR + (i / 8);
                 v_target = v->domain->vcpu[new_target];
                 v_old = v->domain->vcpu[old_target];
                 vgic_migrate_irq(v_old, v_target, irq);
@@ -393,7 +393,7 @@ static int vgic_v2_distr_mmio_write(struct vcpu *v, mmio_info_t *info,
                                              DABT_WORD)] = target;
         else
             vgic_byte_write(&rank->v2.itargets[REG_RANK_INDEX(8,
-                      gicd_reg - GICD_ITARGETSR, DABT_WORD)], target, gicd_reg);
+                      gicd_reg - GICD_ITARGETSR, DABT_WORD)], r, gicd_reg);
         vgic_unlock_rank(v, rank, flags);
         return 1;
     }
-- 
2.1.4