[Qemu-devel] [Bug 1809546] Re: Writing a byte to a pl011 SFR overwrites the whole SFR

[Daniels Umanovskis]

Thanks for the response.

I don't think section 3.1 applies to 8-bit accesses. That is
specifically about reserved locations, and neither offset 0x38 nor 0x39
are reserved, so I think it's a matter of whether 32-bit access is
required or not.

>From what I usually see in ARM documentation, 32-bit access is
explicitly mentioned when required. For the PL011, it's mentioned for
the UARTPeriphID_n registers, for instance. In many other cases access
size depends on the implementation and the corresponding memory mapping
of that implementation.

I understand that *in practice* you should ensure single-access writes
unless doing otherwise is explicitly allowed. However, in cases like the
PL011 it seems ambiguous whether that is actually required, so it seems
like the best choice would be to explicitly document it for the QEMU
implementation. That would save some guesswork.

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Title:
  Writing a byte to a pl011 SFR overwrites the whole SFR

Status in QEMU:
  New

Bug description:
  The bug is present in QEMU 2.8.1 and, if my analysis is correct, also
  on master.

  I first noticed that a PL011 UART driver, which is fine on real
  hardware, fails to enable the RX interrupt in the IMSC register when
  running in QEMU. However, the problem only comes up if the code is
  compiled without optimizations. I think I've narrowed it down to a
  minimal example that will exhibit the problem if run as a bare-metal
  application.

  Given:

  pl011_addr: .word 0x10009000

  The following snippet will be problematic:

       ldr r3, pl011_addr
       ldrb r2, [r3, #0x38]        // IMSC
       mov r2, #0
       orr r2, r2, #0x10           // R2 == 0x10
       strb r2, [r3, #0x38]        // Whole word reads correctly after this
       ldrb r2, [r3, #0x39]
       mov r2, #0
       strb r2, [r3, #0x39]        // Problem here! Overwrites offset 0x38 as 
well

  After the first strb instruction, which writes to 0x10009038,
  everything is fine. It can be seen in the QEMU monitor:

  (qemu) xp 0x10009038
  0000000010009038: 0x00000010

  After the second strb instruction, the write to 0x10009039 clears the
  entire word:

  (qemu) xp 0x10009038
  0000000010009038: 0x00000000

  QEMU command-line, using the vexpress-a9 which has the PL011 at
  0x10009000:

  qemu-system-arm -S -M vexpress-a9 -m 32M -no-reboot -nographic
  -monitor telnet:127.0.0.1:1234,server,nowait -kernel pl011-sfr.bin
  -gdb tcp::2159 -serial mon:stdio

  Compiling the original C code with optimizations makes the driver
  work. It compiles down to assembly that only does a single write:

      ldr r3, pl011_addr
      mov r2, #0x10
      str r2, [r3, #0x38]

  Attached is the an assembly file, and linkscript, that shows the
  problem, and also includes the working code.

  I haven't debugged inside of QEMU itself but it seems to me that the
  problem is in pl011_write in pl011.c - the functions looks at which
  offset is being written, and then writes the entire SFR that offset
  falls under, which means that changing a single byte will change the
  whole SFR.

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