Re: [patch V6 07/37] x86/entry: Provide helpers for execute on irqstack

[Andy Lutomirski]

On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.
>
> The initial attempt of having the stack switching in inline ASM caused too
> much headache vs. objtool and the unwinder. After analysing the use cases
> it was agreed on that having the stack switch in ASM for the price of an
> indirect call is acceptable as the main users are indirect call heavy
> anyway and the few system vectors which are empty shells (scheduler IPI and
> KVM posted interrupt vectors) can run from the regular stack.
>
> 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.
>

Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>

Have you tested by forcing a stack trace from the IRQ stack and making
sure it unwinds all the way out?