Re: [patch V5 06/38] x86/entry: Provide helpers for execute on irqstack

[Josh Poimboeuf]

On Tue, May 12, 2020 at 11:01:05PM +0200, 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.
> 
> 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.
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20200507161020.783541450@xxxxxxxxxxxxx
> ---
> V5: Moved the actual switch to ASM code

Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

-- 
Josh