From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54929) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1axKJI-0003UL-TS for qemu-devel@nongnu.org; Mon, 02 May 2016 16:19:23 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1axKJ7-0005q4-4B for qemu-devel@nongnu.org; Mon, 02 May 2016 16:19:11 -0400 Received: from mail-lf0-x22c.google.com ([2a00:1450:4010:c07::22c]:33404) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1axKJ5-0005mj-3D for qemu-devel@nongnu.org; Mon, 02 May 2016 16:19:05 -0400 Received: by mail-lf0-x22c.google.com with SMTP id y84so201056257lfc.0 for ; Mon, 02 May 2016 13:18:49 -0700 (PDT) References: <5727B088.1090400@gmail.com> From: Sergey Fedorov Message-ID: <5727B623.2070901@gmail.com> Date: Mon, 2 May 2016 23:18:43 +0300 MIME-Version: 1.0 In-Reply-To: <5727B088.1090400@gmail.com> Content-Type: multipart/alternative; boundary="------------070403090005090900000706" Subject: Re: [Qemu-devel] tcg: How 'CPUState::current_tb' is used? List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: QEMU Developers Cc: =?UTF-8?Q?Alex_Benn=c3=a9e?= , Paolo Bonzini , Richard Henderson , Blue Swirl , Riku Voipio This is a multi-part message in MIME format. --------------070403090005090900000706 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit 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 > 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 --------------070403090005090900000706 Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: 8bit
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 <bellard@c046a42c-6fe2-441c-8c8c-71466251a162>
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
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