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Re: Exception return

From: abhijeet inamdar
Subject: Re: Exception return
Date: Thu, 16 Dec 2021 21:06:49 +0100

If I won't make that change it gives me immediate error as follows:

Taking exception 18 [v7M INVSTATE UsageFault]
...BusFault with BFSR.STKERR
...taking pending nonsecure exception 3
qemu: fatal: Lockup: can't take terminal derived exception (original exception priority -1)


On Thu, Dec 16, 2021 at 9:02 PM abhijeet inamdar <abhijeetinamdar3005@gmail.com> wrote:
I made some changes to the code in qemu : https://github.com/qemu/qemu/blob/stable-6.0/target/arm/cpu.c#L339 . I have commented out the variable vecbase and directly given the address of my vector address(not zero) or where they are located in our binary file. Is that a problem? 


On Thu, Dec 16, 2021 at 7:47 PM Peter Maydell <peter.maydell@linaro.org> wrote:
On Thu, 16 Dec 2021 at 17:44, Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
> Cc'ing qemu-arm developers.
> On 12/16/21 18:31, abhijeet inamdar wrote:
> > Hi,
> >
> > I was running my .elf file on my custom machine(CM3) and was hit with
> > this error when debugging and had the required -d flags and single stepping.
> >
> > IN: Hal_MemFault
> > 0x7004e794:  4770       bx       lr
> >
> > Trace 0: 0x7f4faa471100 [00000000/7004e794/0x312000c1]  Hal_MemFault
> > R00=7004938d R01=00000000 R02=7004938c R03=ffffffff
> > R04=7005e410 R05=10020000 R06=e000eda0 R07=00000000
> > R08=00000010 R09=1003f8e0 R10=e000ed94 R11=00000000
> > R12=ffffffff R13=1003dbe0 R14=fffffff9 R15=7004e794
> > XPSR=01000003 ---- T handler
> > Taking exception 8 [QEMU v7M exception exit]
> > Exception return: magic PC fffffff9 previous exception 3
> > M profile return from interrupt with misaligned PC is UNPREDICTABLE on v7M
> > ...successful exception return
> > DRBAR[6]: 0xa0000000 misaligned to DRSR region size, mask = 0x3fffffff
> > Taking exception 3 [Prefetch Abort]
> > ...with CFSR.IACCVIOL
> > ...taking pending nonsecure exception 3

The logging here tells you what happened:

 * we are in an exception handler
 * we do the bx lr to magic value which is M-profile's exception-return
 * ...but there's something wrong with the return address we pull
   off the stack, because it's not an aligned value (its least
   significant bit is set)
 * moreover, it's an address that corresponds to an MPU region
   that has been misprogrammed (the DRBAR value isn't right);
   the MPU treats such regions as not matching
 * and so we take an instruction access fault

So you have two problems:
 (1) the PC value on the stack that you're trying to return to is wrong
 (2) you're programming the MPU wrong

You need to investigate and fix both.

-- PMM

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