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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