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From:   Siddharth Chandrasekaran <sidcha@amazon.de>
To:     "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        Arnd Bergmann <arnd@arndb.de>
CC:     Siddharth Chandrasekaran <sidcha.dev@gmail.com>,
        Liran Alon <liran@amazon.com>,
        Ioannis Aslanidis <iaslan@amazon.de>,
        <linux-hyperv@vger.kernel.org>, <linux-arch@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>,
        Siddharth Chandrasekaran <sidcha@amazon.de>
Subject: [PATCH] asm-generic/hyperv: Fix struct hv_message_header ordering
Date:   Thu, 29 Jul 2021 15:37:02 +0200
Message-ID: <20210729133702.11383-1-sidcha@amazon.de>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

According to Hyper-V TLFS Version 6.0b, struct hv_message_header members
should be defined in the order:

	message_type, reserved, message_flags, payload_size

but we have it defined in the order:

	message_type, payload_size, message_flags, reserved

that is, the payload_size and reserved members swapped. Due to this mix
up, we were inadvertently causing two issues:

    - The payload_size field has invalid data; it didn't cause an issue
      so far because we are delivering only timer messages which has fixed
      size payloads the guest probably did a sizeof(payload) instead
      relying on the value of payload_size member.

    - The message_flags was always delivered as 0 to the guest;
      fortunately, according to section 13.3.1 message_flags is also
      treated as a reserved field.

Although this is not causing an issue now, it might in future (we are
adding more message types in our VSM implementation) so fix it to
reflect the specification.

Signed-off-by: Siddharth Chandrasekaran <sidcha@amazon.de>
---
 include/asm-generic/hyperv-tlfs.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/hyperv-tlfs.h b/include/asm-generic/hyperv-tlfs.h
index 56348a541c50..a5540e9b171f 100644
--- a/include/asm-generic/hyperv-tlfs.h
+++ b/include/asm-generic/hyperv-tlfs.h
@@ -284,9 +284,9 @@ union hv_port_id {
 /* Define synthetic interrupt controller message header. */
 struct hv_message_header {
 	__u32 message_type;
-	__u8 payload_size;
-	union hv_message_flags message_flags;
 	__u8 reserved[2];
+	union hv_message_flags message_flags;
+	__u8 payload_size;
 	union {
 		__u64 sender;
 		union hv_port_id port;
-- 
2.17.1


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