Re: [PATCH v2] target/i386: Give IRQs a chance when resetting HF_INHIBIT_IRQ_MASK

[Paolo Bonzini]

On Mon, Apr 15, 2024 at 8:50 AM Ruihan Li <lrh2000@pku.edu.cn> wrote:
>
> When emulated with QEMU, interrupts will never come in the following
> loop. However, if the NOP instruction is uncommented, interrupts will
> fire as normal.
>
>         loop:
>                 cli
>                 call do_sti
>                 jmp loop
>
>         do_sti:
>                 sti
>                 # nop
>                 ret
>
> This behavior is different from that of a real processor. For example,
> if KVM is enabled, interrupts will always fire regardless of whether the
> NOP instruction is commented or not. Also, the Intel Software Developer
> Manual states that after the STI instruction is executed, the interrupt
> inhibit should end as soon as the next instruction (e.g., the RET
> instruction if the NOP instruction is commented) is executed.

Thanks, interesting bug!

What do you think about writing this:

>      /* If several instructions disable interrupts, only the first does it.  
> */
>      if (inhibit && !(s->flags & HF_INHIBIT_IRQ_MASK)) {
>          gen_set_hflag(s, HF_INHIBIT_IRQ_MASK);
> -    } else {
> +        inhibit_reset = false;
> +    } else if (!inhibit && (s->flags & HF_INHIBIT_IRQ_MASK)) {
>          gen_reset_hflag(s, HF_INHIBIT_IRQ_MASK);
> +        inhibit_reset = true;
> +    } else {
> +        inhibit_reset = false;
>      }

in a slightly simpler manner:

    inhibit_reset = false;
    if (s->flags & HF_INHIBIT_IRQ_MASK) {
        gen_reset_hflag(s, HF_INHIBIT_IRQ_MASK);
        inhibit_reset = true;
    } else if (inhibit) {
        gen_set_hflag(s, HF_INHIBIT_IRQ_MASK);
    }

No need to submit v3, I can do the change myself when applying.

Paolo