Re: [patch V4 part 5 02/31] x86/entry: Provide helpers for execute on irqstack

Alexandre Chartre
2020-05-11T09:09:47Z

On 5/5/20 3:53 PM, Thomas Gleixner wrote:
> Device interrupt handlers and system vector handlers are executed on the
> interrupt stack. The stack switch happens in the low level assembly entry
> code. This conflicts with the efforts to consolidate the exit code in C to
> ensure correctness vs. RCU and tracing.
>
> As there is no way to move #DB away from IST due to the MOV SS issue, the
> requirements vs. #DB and NMI for switching to the interrupt stack do not
> exist anymore. The only requirement is that interrupts are disabled.
>
> That allows to move the stack switching to C code which simplifies the
> entry/exit handling further because it allows to switch stacks after
> handling the entry and on exit before handling RCU, return to usermode and
> kernel preemption in the same way as for regular exceptions.
>
> That also allows to move the xen hypercall extra magic code and the softirq
> stack switching into C.
>
> The mechanism is straight forward:
>
> 1) Store the current stack pointer on top of the interrupt stack. That's
> required for the unwinder.
>
> 2) Switch the stack pointer
>
> 3) Call the function
>
> 4) Restore the stackpointer
>
> The full code sequence to make the unwinder happy is:
>
> pushq %rbp
> movq %rsp, %rbp
> movq $(top_of_hardirq_stack - 8), %reg
> movq %rsp, (%reg)
> movq %reg , %rsp
> call function
> popq %rsp
> leaveq
>
> While the following sequence would spare the 'popq %rsp':
>
> pushq %rbp
> movq $(top_of_hardirq_stack - 8), %rbp
> movq %rsp, (%rrbp)

Should be (%rbp) instead of (%rrbp).

> xchgq %rbp, %rsp
> call function
> movq %rbp, %rsp
> leaveq
>
> but that requires further changes to objtool so that the unwinder works
> correctly. Can be done on top and is not critical for now.
>
> Provide helper functions to check whether the interrupt stack is already
> active and whether stack switching is required.
>
> 64 bit only for now. 32 bit has a variant of that already. Once this is
> cleaned up the two implementations might be consolidated as a cleanup on
> top.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> arch/x86/include/asm/irq_stack.h | 61 +++++++++++++++++++++++++++++++++++++++
> 1 file changed, 61 insertions(+)
>
> --- /dev/null
> +++ b/arch/x86/include/asm/irq_stack.h
...
> +/*
> + * Macro to emit code for running @func on the irq stack.
> + */
> +#define RUN_ON_IRQSTACK(func) { \
> + unsigned long tos; \
> + \
> + lockdep_assert_irqs_disabled(); \
> + \
> + tos = ((unsigned long)__this_cpu_read(hardirq_stack_ptr)) - 8; \
> + \
> + __this_cpu_add(irq_count, 1); \
> + asm volatile( \
> + "pushq %%rbp \n" \
> + "movq %%rsp, %%rbp \n" \
> + "movq %%rsp, (%[ts]) \n" \
> + "movq %[ts], %%rsp \n" \
> + "1: \n" \
> + " .pushsection .discard.instr_begin \n" \
> + " .long 1b - . \n" \
> + " .popsection \n" \
> + "call " __ASM_FORM(func) " \n" \
> + "2: \n" \
> + " .pushsection .discard.instr_end \n" \
> + " .long 2b - . \n" \
> + " .popsection \n" \
> + "popq %%rsp \n" \
> + "leaveq \n" \
> + : \
> + : [ts] "r" (tos) \
> + : "memory" \
> + ); \
> + __this_cpu_sub(irq_count, 1); \
> +}

The pushsection/popsection discard.instr_begin/end sequences are used several
times in asm() statement at different places, so I wonder if it might be worth
having a macro.

In part 1, patch 20/36 adds instr_begin()/end(): they provide the sequence
but already encapsulated into an asm() statement, then we could do something
like this:

/* Begin/end of an instrumentation safe region */
#define instr_begin_insn(label) \
__stringify(label) ":\n\t" \
".pushsection .discard.instr_begin\n\t" \
".long " __stringify(label) "b - .\n\t" \
".popsection\n\t"

#define instr_end_insn(label) \
__stringify(label) ":\n\t" \
".pushsection .discard.instr_end\n\t" \
".long " __stringify(label) "b - .\n\t" \
".popsection\n\t"

#define instr_begin() ({asm volatile(instr_begin_insn(__COUNTER__));})
#define instr_end() ({asm volatile(instr_end_insn(__COUNTER__));})> +#else /* CONFIG_X86_64 */

And the RUN_ON_IRQSTACK macro would become:

#define RUN_ON_IRQSTACK(func) { \
unsigned long tos; \
\
lockdep_assert_irqs_disabled(); \
\
tos = ((unsigned long)__this_cpu_read(hardirq_stack_ptr)) - 8; \
\
__this_cpu_add(irq_count, 1); \
asm volatile( \
"pushq %%rbp \n" \
"movq %%rsp, %%rbp \n" \
"movq %%rsp, (%[ts]) \n" \
"movq %[ts], %%rsp \n" \
instr_begin_insn(1) \
"call " __ASM_FORM(func) " \n" \
instr_end_insn(2) \
"popq %%rsp \n" \
"leaveq \n" \
: \
: [ts] "r" (tos) \
: "memory" \
); \
__this_cpu_sub(irq_count, 1); \
}

alex.