arm64: rsi: Correct type of realm_config::hash_algo

The RMM specification (DEN0137, B5.5.10 and B5.5.19) defines
RsiHashAlgorithm as an 8-bit enumeration, stored at offset 0x8 of
RsiRealmConfig, with the unused bits of the structure being MBZ.
However, realm_config::hash_algo is declared as unsigned long.

For an RMM that zeroes the unused bits, the wider type gives the same
value. An RMM that only writes the low byte (as TF-RMM currently does)
leaves bytes 1-7 with whatever the guest buffer contained, so a
comparison of the 64-bit field against RSI_HASH_SHA_* can fail. Nothing
in mainline reads the field yet, but the proposed Realm measurement
register support does.

Declare the field as u8 so the guest only depends on the byte the
specification defines. The struct size and the offsets of the other
fields do not change.

Link: https://lore.kernel.org/all/20260413084957.327661-4-sami.mujawar@arm.com/
Fixes: b880a80011f5 ("arm64: rsi: Add RSI definitions")
Signed-off-by: Kohei Enju <enju.kohei@fujitsu.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/arm-smccc-rsi.h b/include/linux/arm-smccc-rsi.h
index fddb77986f70..cca881ddfdba 100644
--- a/include/linux/arm-smccc-rsi.h
+++ b/include/linux/arm-smccc-rsi.h
@@ -128,7 +128,7 @@ struct realm_config {
 	union {
 		struct {
 			unsigned long ipa_bits; /* Width of IPA in bits */
-			unsigned long hash_algo; /* Hash algorithm */
+			u8 hash_algo; /* Hash algorithm */
 		};
 		u8 pad[0x200];
 	};