From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:55368)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <peter.maydell@linaro.org>) id 1YlEp4-0000YW-OU
	for qemu-devel@nongnu.org; Thu, 23 Apr 2015 06:57:35 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <peter.maydell@linaro.org>) id 1YlEp1-00046r-HA
	for qemu-devel@nongnu.org; Thu, 23 Apr 2015 06:57:34 -0400
Received: from mail-ig0-f173.google.com ([209.85.213.173]:34203)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <peter.maydell@linaro.org>) id 1YlEp1-00046n-CT
	for qemu-devel@nongnu.org; Thu, 23 Apr 2015 06:57:31 -0400
Received: by iget9 with SMTP id t9so7995567ige.1
	for <qemu-devel@nongnu.org>; Thu, 23 Apr 2015 03:57:30 -0700 (PDT)
MIME-Version: 1.0
In-Reply-To: <CAJy5ezqzHVwgHfmkxSDk4yDZb-6zoiU-cxeBNkUWSAF8Xj4GjQ@mail.gmail.com>
References: <1429722561-12651-1-git-send-email-greg.bellows@linaro.org>
	<1429722561-12651-10-git-send-email-greg.bellows@linaro.org>
	<20150423024929.GC17116@toto>
	<CAFEAcA_0j3mtqrORwV38sTCyYk7BWQPi0EmGks60MRqeYO=D3Q@mail.gmail.com>
	<CAJy5ezqzHVwgHfmkxSDk4yDZb-6zoiU-cxeBNkUWSAF8Xj4GjQ@mail.gmail.com>
From: Peter Maydell <peter.maydell@linaro.org>
Date: Thu, 23 Apr 2015 11:57:10 +0100
Message-ID: <CAFEAcA8QXQVH1VSWP=xuY10oAda2-g7oESsjXv682jBGYNwpUw@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Subject: Re: [Qemu-devel] [PATCH v2 9/9] target-arm: Add WFx instruction
	trap support
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Cc: Sergey Fedorov <serge.fdrv@gmail.com>, =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>, QEMU Developers <qemu-devel@nongnu.org>, Greg Bellows <greg.bellows@linaro.org>

On 23 April 2015 at 11:39, Edgar E. Iglesias <edgar.iglesias@gmail.com> wrote:
>
> On 23/04/2015 6:00 pm, "Peter Maydell" <peter.maydell@linaro.org> wrote:
>> In theory you could maybe check has_work() for the WFI case,
>> since doing an EXCP_HLT really should cause us to stop until
>> has_work is true, but it seems a bit fragile -- could we really
>> guarantee that nothing would change between this point and
>> when we went back through the main loop that would change
>> whether has_work evaluates true or not? I think that it's better
>> there too to just always take the trap: setting EXCP_HLT is our
>> "going into a low power state" and so we should take the trap
>> if we would otherwise have done that.
>
> I think functional wise we are OK.
> The implementation can AFAIK always choose to nop for whatever reason (e.g
> has_work()). Only when we choose to enter low power, the trap comes into
> play.

Ah, so in helper_wfi() do something like

    if (!has_work()) {
       if (trapping wfi) {
           EXCP_UDEF code;
       } else {
           EXCP_HALT code;
       }
    }
    /* otherwise just return, making this WFI a nop */

?

I think that would work.

> Maybe wfe is the most problematic one because it fires more frequently and
> often when has_work() is true?

Yes, I think we should start by not trapping on WFE and then look
at how good/bad perf is.

-- PMM