Re: [patch V2 07/17] x86/entry/64: Remove redundant interrupt disable

From: Peter Zijlstra
Date: Thu Oct 24 2019 - 17:21:36 EST

On Thu, Oct 24, 2019 at 10:52:59PM +0200, Thomas Gleixner wrote:
> is just undefined behaviour and I personally consider it to be a plain bug.

I concur.

> Just for the record: This results in running a resulting or even completely
> unrelated signal handler with interrupts disabled as well.
> Whatever we decide it is, leaving it completely inconsistent is not a
> solution at all. The options are:
> 1) Always do conditional tracing depending on the user_regs->eflags.IF
> state.
> 2) #1 + warn once when syscalls and exceptions (except NMI/MCE) happen
> and user_regs->eflags.IF is cleared.
> 3a) #2 + enforce signal handling to run with interrupts enabled.
> 3b) #2 + set regs->eflags.IF. So the state is always correct from the
> kernel POV. Of course that changes existing behaviour, but its
> changing undefined and inconsistent behaviour.
> 4) Let iopl(level) return -EPERM if level == 3.
> Yeah, I know it's not possible due to regressions (DPKD uses iopl(3)),
> but TBH that'd be the sanest option of all.
> Of course the infinite wisdom of hardware designers tied IN, INS, OUT,
> OUTS and CLI/STI together on IOPL so we cannot even distangle them in
> any way.
> The only way out would be to actually use a full 8K sized I/O bitmap,
> but that's a massive pain as it has to be copied on every context
> switch.
> Really pretty options to chose from ...

If 4 is out (and I'm afraid it might be), then I'm on record for liking