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

[Oleksiy Bondarenko]

Thanks for clarification. Now I understood what you there talking about
PopStack pseudocode function.

(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.

It seams true because I have used a lot of different cortex m3 micros  and none 
of them does not refused to execute such task stack initialization. I have  
just looked in Micrium uCOS III sources and in there there is the same usage of 
THUMB pointer to init PC.
My point that in production there is a lot of code that does not care about 
this PREDICTABLE behavior and in my case I just want to run firmware without 
recompile and changes as it is. And for those who want this it is better at 
least to mention this behavior in comments or some there else , and maybe this 
will save a lot of time.

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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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