[Qemu-devel] [Bug 1713066] [NEW] Incorrect handling of aarch64 ldp in some cases

[Andrew <andrew@fubar.geek.nz>]

Public bug reported:

In some cases the ldp instruction (and presumably other multi-register
loads and stores) can behave incorrectly.

Given the following instruction:
ldp x0, x1, [x0]

This will load two 64 bit values from memory, however if each location
to load is on a different page and the second page is unmapped this will
raise an exception. When this happens x0 has already been updated so
after the exception handler has run the operating system will try to
rerun the instruction. QEMU will now try to perform an invalid load and
raise a new exception.

I believe this is incorrect as section D.1.14.5 of the ARMv8 reference
manual B.a states that, on taking an exception, registers used in the
generation of addresses are restored to their initial value, so x0
shouldn't be changed, where x1 can be un an unknown state.

I found the issue running FreeBSD with the cortex-strings implementation
of memcpy. This uses a similar instruction when copying between 64 and
96 bytes.

I've observed this on:
QEMU emulator version 2.5.0 (Debian 1:2.5+dfsg-5ubuntu10.14), Copyright (c) 2003-2008 Fabrice Bellard

And checked I still get the same behaviour on:
QEMU emulator version 2.9.94 (v2.10.0-rc4-dirty)
Git revision: 248b23735645f7cbb503d9be6f5bf825f2a603ab

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1713066

Title:
  Incorrect handling of aarch64 ldp in some cases

Status in QEMU:
  New

Bug description:
  In some cases the ldp instruction (and presumably other multi-register
  loads and stores) can behave incorrectly.

  Given the following instruction:
  ldp x0, x1, [x0]

  This will load two 64 bit values from memory, however if each location
  to load is on a different page and the second page is unmapped this
  will raise an exception. When this happens x0 has already been updated
  so after the exception handler has run the operating system will try
  to rerun the instruction. QEMU will now try to perform an invalid load
  and raise a new exception.

  I believe this is incorrect as section D.1.14.5 of the ARMv8 reference
  manual B.a states that, on taking an exception, registers used in the
  generation of addresses are restored to their initial value, so x0
  shouldn't be changed, where x1 can be un an unknown state.

  I found the issue running FreeBSD with the cortex-strings
  implementation of memcpy. This uses a similar instruction when copying
  between 64 and 96 bytes.

  I've observed this on:
  QEMU emulator version 2.5.0 (Debian 1:2.5+dfsg-5ubuntu10.14), Copyright (c) 2003-2008 Fabrice Bellard

  And checked I still get the same behaviour on:
  QEMU emulator version 2.9.94 (v2.10.0-rc4-dirty)
  Git revision: 248b23735645f7cbb503d9be6f5bf825f2a603ab

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