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From: Adam Lackorzynski <adam@l4re.org>
To: qemu-devel@nongnu.org, Peter Maydell <peter.maydell@linaro.org>
Subject: arm: Launching EFI-enabled arm32 Linux
Date: Sat, 4 Sep 2021 21:26:45 +0200	[thread overview]
Message-ID: <YTPIdbUCmwagL5/D@os.inf.tu-dresden.de> (raw)

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


             reply	other threads:[~2021-09-04 19:27 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-04 19:26 Adam Lackorzynski [this message]
2021-09-05 14:44 ` arm: Launching EFI-enabled arm32 Linux Peter Maydell
2021-09-06 15:34 ` Andre Przywara
2021-09-07 23:25   ` Adam Lackorzynski
2021-09-07 23:47     ` Andre Przywara
2021-09-08 11:04       ` Adam Lackorzynski

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