From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:59309)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <peter.maydell@linaro.org>) id 1WPX7j-0006uP-EF
	for qemu-devel@nongnu.org; Mon, 17 Mar 2014 08:58:39 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <peter.maydell@linaro.org>) id 1WPX7d-0000OA-2u
	for qemu-devel@nongnu.org; Mon, 17 Mar 2014 08:58:35 -0400
Received: from mail-lb0-f169.google.com ([209.85.217.169]:55155)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <peter.maydell@linaro.org>) id 1WPX7c-0000Nv-Rc
	for qemu-devel@nongnu.org; Mon, 17 Mar 2014 08:58:28 -0400
Received: by mail-lb0-f169.google.com with SMTP id q8so3716414lbi.28
	for <qemu-devel@nongnu.org>; Mon, 17 Mar 2014 05:58:27 -0700 (PDT)
MIME-Version: 1.0
In-Reply-To: <CAEgOgz5=-RRQj4r0D2dgAvDvKsFi44v_VdBXbuREOGAjJ2aoHQ@mail.gmail.com>
References: <1394134385-1727-1-git-send-email-peter.maydell@linaro.org>
	<1394134385-1727-12-git-send-email-peter.maydell@linaro.org>
	<CAEgOgz5=-RRQj4r0D2dgAvDvKsFi44v_VdBXbuREOGAjJ2aoHQ@mail.gmail.com>
From: Peter Maydell <peter.maydell@linaro.org>
Date: Mon, 17 Mar 2014 12:58:07 +0000
Message-ID: <CAFEAcA-QFQz43vAxib8d-v5eMTSDM5duSt1=bJbk=ZaJY13RZA@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Subject: Re: [Qemu-devel] [PATCH v4 11/21] target-arm: Don't mention PMU in
 debug feature register
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: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Cc: Rob Herring <rob.herring@linaro.org>, Patch Tracking <patches@linaro.org>, Michael Matz <matz@suse.de>, Claudio Fontana <claudio.fontana@huawei.com>, Alexander Graf <agraf@suse.de>, "qemu-devel@nongnu.org Developers" <qemu-devel@nongnu.org>, Laurent Desnogues <laurent.desnogues@gmail.com>, Dirk Mueller <dmueller@suse.de>, Will Newton <will.newton@linaro.org>, =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>, "kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>, Christoffer Dall <christoffer.dall@linaro.org>, Richard Henderson <rth@twiddle.net>

On 17 March 2014 05:13, Peter Crosthwaite <peter.crosthwaite@xilinx.com> wrote:
> On Fri, Mar 7, 2014 at 5:32 AM, Peter Maydell <peter.maydell@linaro.org> wrote:
>> Suppress the ID_AA64DFR0_EL1 PMUVer field, even if the CPU specific
>> value claims that it exists. QEMU doesn't currently implement it,
>> and not advertising it prevents the guest from trying to use it
>> and getting UNDEFs on unimplemented registers.
>>
>> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
>
> Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
>
>> ---
>> This is arguably a hack, but otherwise Linux tries to prod
>> half a dozen PMU sysregs.
>
> Not really. I think sane self-identification trumps dummy feature
> advertising. Although there is a consistency argument to be made, as
> to whether you should also wipe-out any other features advertised by
> this register and friends (e.g. should TraceVer be set?).

The lack of consistency is what makes it a hack :-) Generally
QEMU takes the approach of "report what the h/w reports even
if we don't implement it all"; "report what we provide even
if that's not the same values as h/w" would be a different
approach, but if we wanted that we'd need to do it consistently.
Still I think pragmatism wins out here.

thanks
-- PMM