[Qemu-devel] [Bug 657006] Re: arm v7M - svc insn doesn't trigger PendSV handler

[Peter Maydell]

(1) You should be looking at DDI0403D -- revision D of the v7M ARM ARM
included some significant clarifications and corrections as well as
adding documentation of floating point support.

(2) The behaviour of the POP instruction is irrelevant here because the
QEMU function you are proposing to change is not related to it but is in
fact implementing the exception return handling. As I said before, this
corresponds to the PopStack pseudocode function in the v7m ARM, which is
clearly documented as UNPREDICTABLE if the lsbit is set.

(3) Joseph Yiu's book (however good it may be) is not the authoritative
reference to the behaviour that v7M software can rely on; that is the
architecture reference manual.

(4) It is entirely possible that hardware implementations to date ignore
the lsbit in this situation. That doesn't mean that software which
relies on this UNPREDICTABLE behaviour is not buggy.

(5) The code in
http://freertos.svn.sourceforge.net/viewvc/freertos/trunk/Source/portable/GCC/ARM_CM3/port.c?revision=1660&view=markup
pxPortInitialiseStack() is wrong because it does not force the lsbit to
zero when setting up its fake stack frame. That (or the equivalent in
whichever freertos port you're building) is what you need to fix.

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https://bugs.launchpad.net/bugs/657006

Title:
  arm v7M - svc insn doesn't trigger PendSV handler

Status in QEMU:
  New

Bug description:
  The svc instruction doesn't work as expected.

  -> qemu 0.13.0 rc1 (git)

  Test : demo with freeRTOS (for example
  FreeRTOS-6.0.5/Demo/CORTEX_LM3S811_GCC) with the card lm3s811evb.

  If we start the scheduler, it will call that function (__attribute__
  (( naked ))) :

  void vPortStartFirstTask( void )

  {

          __asm volatile(

                                          " ldr r0, =0xE000ED08   \n" /*
  Use the NVIC offset register to locate the stack. */

                                          " ldr r0, [r0]
  \n"

                                          " ldr r0, [r0]
  \n"

                                          " msr msp, r0
  \n" /* Set the msp back to the start of the stack. */

                                          " svc 0
  \n" /* System call to start first task. */

                                  );

  }

  The 4 first lines in asm work fine. The scv 0 call will rise the right 
interrupt in qemu (line 151, in arm_gic.c, best_irq = 15). However, it will 
never call the PendSV Handler (xPortPendSVHandler here). This function is 
recorded in the nvic vector.
  Next, (after the svc), the processor will execute the line after in code 
(this is a naked function) so the next function written after 
vPortStartFirstTask in the code.

  
  command line :
  console 1 : qemu-system-arm -M lm3s6965evb -kernel gcc/RTOSDemo.axf -s -S
  console 2 : arm-none-eabi-gdb -ex "target remote localhost:1234" 
gcc/RTOSDemo.axf

  arm-none-eabi from 
http://www.codesourcery.com/sgpp/lite/arm/portal/release1294
  Same error with another project with arm-elf

  processor : arm cortex m3

  host : gentoo (2.6.35-r9) (without kqemu)

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