We expect mmio read handlers to leave the bits above the access size
zeroed. Add an ASSERT to check this aspect of read handler behavior.

Suggested-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
Acked-by: Julien Grall <jgrall@amazon.com>
---
There's no need to wait for the rest of the series to commit this patch.

v18->v19:
* add Julien's A-b
* s/GENMASK_ULL/GENMASK/

v17->v18:
* no change

v16->v17:
* new patch

See https://lore.kernel.org/xen-devel/bc6660ef-59f1-4514-9792-067d987e3fbc@xen.org/

Also see 7db7bd0f319f ("arm/vpci: honor access size when returning an error")

Also see xen/arch/arm/ioreq.c:handle_ioserv()
---
 xen/arch/arm/io.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
index 653428e16c1f..5a4b0e8f25c6 100644
--- a/xen/arch/arm/io.c
+++ b/xen/arch/arm/io.c
@@ -37,6 +37,8 @@ static enum io_state handle_read(const struct mmio_handler *handler,
     if ( !handler->ops->read(v, info, &r, handler->priv) )
         return IO_ABORT;
 
+    ASSERT((r & ~GENMASK((1U << info->dabt.size) * 8 - 1, 0)) == 0);
+
     r = sign_extend(dabt, r);
 
     set_user_reg(regs, dabt.reg, r);
-- 
2.49.0