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instruction: 0.765
runtime: 0.152
syscall: 0.084
Incorrect handling of aarch64 ldp in some cases
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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