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[PATCH] hyperv: Fix struct hv_message_header ordering

From: Siddharth Chandrasekaran
Subject: [PATCH] hyperv: Fix struct hv_message_header ordering
Date: Thu, 29 Jul 2021 15:52:10 +0200

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/hw/hyperv/hyperv-proto.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/hw/hyperv/hyperv-proto.h b/include/hw/hyperv/hyperv-proto.h
index 21dc28aee9..f578a60e78 100644
--- a/include/hw/hyperv/hyperv-proto.h
+++ b/include/hw/hyperv/hyperv-proto.h
@@ -101,9 +101,9 @@ struct hyperv_signal_event_input {
 struct hyperv_message_header {
     uint32_t message_type;
-    uint8_t  payload_size;
-    uint8_t  message_flags; /* HV_MESSAGE_FLAG_XX */
     uint8_t  _reserved[2];
+    uint8_t  message_flags; /* HV_MESSAGE_FLAG_XX */
+    uint8_t  payload_size;
     uint64_t sender;

Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879

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