[Qemu-devel] [Bug 1830872] Re: AARCH64 to ARMv7 mistranslation in TCG

[Philippe Mathieu-Daudé]

** Tags added: arm

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Title:
  AARCH64 to ARMv7 mistranslation in TCG

Status in QEMU:
  New

Bug description:
  The following guest code:

  
https://github.com/tianocore/edk2/blob/3604174718e2afc950c3cc64c64ba5165c8692bd/MdePkg/Library/BaseMemoryLibOptDxe/AArch64/CopyMem.S

  implements, in hand-optimized aarch64 assembly, the CopyMem() edk2 (EFI
  Development Kit II) library function. (CopyMem() basically has memmove()
  semantics, to provide a standard C analog here.) The relevant functions
  are InternalMemCopyMem() and __memcpy().

  When TCG translates this aarch64 code to x86_64, everything works
  fine.

  When TCG translates this aarch64 code to ARMv7, the destination area of
  the translated CopyMem() function becomes corrupted -- it differs from
  the intended source contents. Namely, in every 4096 byte block, the
  8-byte word at offset 4032 (0xFC0) is zeroed out in the destination,
  instead of receiving the intended source value.

  I'm attaching two hexdumps of the same destination area:

  - "good.txt" is a hexdump of the destination area when CopyMem() was
    translated to x86_64,

  - "bad.txt" is a hexdump of the destination area when CopyMem() was
    translated to ARMv7.

  In order to assist with the analysis of this issue, I disassembled the
  aarch64 binary with "objdump". Please find the listing in
  "DxeCore.objdump", attached. The InternalMemCopyMem() function starts at
  hex offset 2b2ec. The __memcpy() function starts at hex offset 2b180.

  And, I ran the guest on the ARMv7 host with "-d
  in_asm,op,op_opt,op_ind,out_asm". Please find the log in
  "tcg.in_asm.op.op_opt.op_ind.out_asm.log", attached.

  The TBs that correspond to (parts of) the InternalMemCopyMem() and
  __memcpy() functions are scattered over the TCG log file, but the offset
  between the "nice" disassembly from "DxeCore.objdump", and the in-RAM
  TBs in the TCG log, can be determined from the fact that there is a
  single prfm instruction in the entire binary. The instruction's offset
  is 0x2b180 in "DxeCore.objdump" -- at the beginning of the __memcpy()
  function --, and its RAM address is 0x472d2180 in the TCG log. Thus the
  difference (= the load address of DxeCore.efi) is 0x472a7000.

  QEMU was built at commit a4f667b67149 ("Merge remote-tracking branch
  'remotes/cohuck/tags/s390x-20190521-3' into staging", 2019-05-21).

  The reproducer command line is (on an ARMv7 host):

    qemu-system-aarch64 \
      -display none \
      -machine virt,accel=tcg \
      -nodefaults \
      -nographic \
      -drive 
if=pflash,format=raw,file=$prefix/share/qemu/edk2-aarch64-code.fd,readonly \
      -drive 
if=pflash,format=raw,file=$prefix/share/qemu/edk2-arm-vars.fd,snapshot=on \
      -cpu cortex-a57 \
      -chardev stdio,signal=off,mux=on,id=char0 \
      -mon chardev=char0,mode=readline \
      -serial chardev:char0

  The apparent symptom is an assertion failure *in the guest*, such as

  > ASSERT [DxeCore]
  > 
/home/lacos/src/upstream/qemu/roms/edk2/MdePkg/Library/BaseLib/String.c(1090):
  > Length < _gPcd_FixedAtBuild_PcdMaximumAsciiStringLength

  but that is only a (distant) consequence of the CopyMem()
  mistranslation, and resultant destination area corruption.

  Originally reported in the following two mailing list messages:
  - http://mid.mail-archive.com/address@hidden
  - http://mid.mail-archive.com/address@hidden

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