arm: Launching EFI-enabled arm32 Linux

[Adam Lackorzynski]

Hi,

while trying to launch an EFI-enabled arm32 Linux binary (zImage) I
noticed I get an undefined instruction exception on the first
instruction. Now this is a bit special because Linux uses a nop
instruction there that also is a PE file signature ('MZ') such that the
CPU runs over it and the file is still recognized as a PE binary. Linux
uses 0x13105a4d (tstne r0, #0x4d000) as the instruction (see also
arch/arm/boot/compressed/head.S and efi-header.S in Linux).
However, QEMU's instruction decoder will only recognize TST with bits
12-15 being 0, which this instruction is not fullfilling, and thus the
undef exception. I guess other CPU implementations will allow this
encoding. So while investigating I was doing the following to make Linux
proceed. I also believe this was working in a previous version of QEMU.

diff --git a/target/arm/a32.decode b/target/arm/a32.decode
index fcd8cd4f7d..222553750e 100644
--- a/target/arm/a32.decode
+++ b/target/arm/a32.decode
@@ -127,7 +127,7 @@ ADD_rri          .... 001 0100 . .... .... ............     
  @s_rri_rot
 ADC_rri          .... 001 0101 . .... .... ............       @s_rri_rot
 SBC_rri          .... 001 0110 . .... .... ............       @s_rri_rot
 RSC_rri          .... 001 0111 . .... .... ............       @s_rri_rot
-TST_xri          .... 001 1000 1 .... 0000 ............       @S_xri_rot
+TST_xri          .... 001 1000 1 .... ---- ............       @S_xri_rot
 TEQ_xri          .... 001 1001 1 .... 0000 ............       @S_xri_rot
 CMP_xri          .... 001 1010 1 .... 0000 ............       @S_xri_rot
 CMN_xri          .... 001 1011 1 .... 0000 ............       @S_xri_rot


Any thoughts on this?



Adam