[PATCH 1/8] ARM: ARMv7-M: Fix register restore corrupt after svc call

[dillon min]

Hi Vladimir,

Thanks for the review.

On Wed, Mar 3, 2021 at 5:52 PM Vladimir Murzin <vladimir.murzin at arm.com> wrote:
>
> On 3/3/21 8:05 AM, dillon.minfei at gmail.com wrote:
> > From: dillon min <dillon.minfei at gmail.com>
> >
> > For some case, kernel not boot by u-boot(single thread),
> > but by rtos , as most rtos use pendsv to do context switch.
>
>
> Hmm, does it mean that it starts kernel from process context?
   Yes, kernel might be started from process context, since u-boot not
switch context, so kernel always startup under msp.
>
> I'd assume that it is not only kernel who expects MSP. So, what
> if RTOS you mentioned want to boot other RTOS (even itself)? What
> if you have no access to the source code for those RTOS(es) to
> patch MSP/PSP switch?

My case is a little complicated.
stm32h7 only have 128Kbytes internal flash, can't store u-boot.bin (>200K),
so, set a bootloader (rt-thread rtos) to internal flash, load
linux/u-boot from serial port via ymodem
store to qspi flash(8Mbytes), then jump to u-boot.

qspi flash layout:
0 - 512K:    u-boot
512K- 8M : kernel(xip)

load process : rt-thread -> u-boot -> linux

before add psp/msp check after svc call, register restore corrupt.
add a printhex8 around svc call, found the sp stack is 0x24040000c0ffcff8
it should be 0xc0ffcdf8c0ffcff8. 0x24040000 is the sp stack address
assigned by u-boot
i've no idea how it's become to u-boot's sp.

I have the rtos code, and will try to fix it on the rtos side.

Can you give more explanation about why linux relies on MSP ? thanks

>
> I'd very much prefer to keep stack switching logic outside kernel,
> say, in some shim which RTOS/bootloader can maintain.
>
> Cheers
> Vladimir
>
> >
> > So, we need add an lr check after svc call, to find out should
> > use psp or msp. else register restore after svc call might be
> > corrupted.
> >
> > Fixes: b70cd406d7fe ("ARM: 8671/1: V7M: Preserve registers across switch from Thread to Handler mode")
> > Signed-off-by: dillon min <dillon.minfei at gmail.com>
> > ---
> >  arch/arm/mm/proc-v7m.S | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm/mm/proc-v7m.S b/arch/arm/mm/proc-v7m.S
> > index 84459c1d31b8..c93d2757312d 100644
> > --- a/arch/arm/mm/proc-v7m.S
> > +++ b/arch/arm/mm/proc-v7m.S
> > @@ -137,7 +137,10 @@ __v7m_setup_cont:
> >  1:   cpsid   i
> >       /* Calculate exc_ret */
> >       orr     r10, lr, #EXC_RET_THREADMODE_PROCESSSTACK
> > -     ldmia   sp, {r0-r3, r12}
> > +     tst     lr, #EXC_RET_STACK_MASK
> > +     mrsne   r4, psp
> > +     moveq   r4, sp
> > +     ldmia   r4!, {r0-r3, r12}
> >       str     r5, [r12, #11 * 4]      @ restore the original SVC vector entry
> >       mov     lr, r6                  @ restore LR
> >
> >
>