On Wed, Oct 21, 2020 at 01:45:42PM +0100, Russell King - ARM Linux admin wrote:

On Wed, Oct 21, 2020 at 02:42:48PM +0200, Arnd Bergmann wrote:

quoted

(also resending this reply from @kernel.org)

On Fri, Oct 16, 2020 at 12:19 PM Arnd Bergmann [off-list ref] wrote:

quoted

On Thu, Oct 8, 2020 at 9:20 AM Maninder Singh [off-list ref] wrote:

quoted

This patch adds code for switching to IRQ stack.
IRQ stack and Kernel SVC stack have below design.

IRQ STACK:-
                    ------------ IRQ stack top
                    |          |
                    ------------
                    .          .
                    .          .
                    .          .
                    ------------
                    |    sp    | <- irq_stack_base + 0x8
                    ------------
                    |    fp    | <- irq_stack_base + 0x4
                    ------------
                    |tinfo_ptr | /* pointer to thread info */
irq_stack_ptr -->   ------------ IRQ stack base

Kernel SVC stack:-
                    ------------  Kernel stack top
                    |          |
                    ------------
                    .          .
                    .          .
                    .          .
                    ------------
                    |          |
                    |          |
                    ------------
                    |tinfo_ptr |  /* pointer to thread info */
                    ------------ Kernel stack base

The extra indirection doesn't look great, and I don't see any of the
other architectures need that. Since we can access percpu data
without going through thread_info, maybe doing the same as
x86 would work here:

- define 'current' as 'this_cpu_read_stable(current_task);'
- convert to CONFIG_THREAD_INFO_IN_TASK

That means we need to also code that up in assembly - remember, we
need to access thread_info from assembly code.

Note also that there is a circular dependency involved. If you make
thread_info accessible via per-cpu, then:

#ifndef __my_cpu_offset
#define __my_cpu_offset per_cpu_offset(raw_smp_processor_id())
#endif
#ifdef CONFIG_DEBUG_PREEMPT
#define my_cpu_offset per_cpu_offset(smp_processor_id())
#else
#define my_cpu_offset __my_cpu_offset
#endif

smp_processor_id() ultimately ends up as raw_smp_processor_id() which
is:

#define raw_smp_processor_id() (current_thread_info()->cpu)

and if current_thread_info() itself involves reading from per-cpu data,
we end up recursing... infinitely.

This is why I said in the other thread:

"We don't do it because we don't have a separate register to be able
to store the thread_info pointer, and copying that lump between the
SVC and IRQ stack will add massively to IRQ latency, especially for
older machines."

-- 
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