[xen staging] xen/arm: check read handler behavior

[patchbot@xxxxxxx]

commit 9a5e22b64266c19b2b01c47a6728be42a8195e52
Author:     Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
AuthorDate: Fri Apr 18 14:58:36 2025 -0400
Commit:     Stefano Stabellini <stefano.stabellini@xxxxxxx>
CommitDate: Mon May 5 14:38:59 2025 -0700

    xen/arm: check read handler behavior
    
    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@xxxxxxxxxx>
    Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
    Acked-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 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 653428e16c..5a4b0e8f25 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);
--
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