Re: [Qemu-devel] tcg: How 'CPUState::current_tb' is used?

[Sergey Fedorov]

On 02/05/16 22:54, Sergey Fedorov wrote:

Hi,

I can't figure out how this field is used. The comment says it's "Currently executing TB", but actually it's the first TB in a chain of TBs executed. Grep shows the only place it is really checked is tb_invalidate_phys_page_range(). That code seems to be introduced long ago in:

commit ea1c18022edd0e2c45552d6fc2da6e15a3486b33
Author: bellard <address@hidden>
Date:   Mon Jun 14 18:56:36 2004 +0000

    fixed self modifying code in case of asynchronous interrupt

I suspect it's only related to user emulation. But I would appreciate if someone could give me an idea of how this really works :)

UPD: 'CPUState::current_tb' was used in that version of QEMU by this code:

/* mask must never be zero, except for A20 change call */
void cpu_interrupt(CPUState *env, int mask)
{              
    TranslationBlock *tb;
    static int interrupt_lock;
                         
    env->interrupt_request |= mask;
    /* if the cpu is currently executing code, we must unlink it and
       all the potentially executing TB */
    tb = env->current_tb;
    if (tb && !testandset(&interrupt_lock)) {
        env->current_tb = NULL;
        tb_reset_jump_recursive(tb);
        interrupt_lock = 0;
    }
}

cpu_interrupt() has changed almost completely since that time. I'm wondering if checking 'cpu->current_tb' by this code in tb_invalidate_phys_page_range() still makes any sense:

if (cpu->interrupt_request && cpu->current_tb) {
    cpu_interrupt(cpu, cpu->interrupt_request);
}

BTW, I'm not sure about the purpose of this piece of code either :)

Kind regards,
Sergey