From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:50229)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1V3nnl-0008Oh-Dt
	for qemu-devel@nongnu.org; Mon, 29 Jul 2013 09:47:59 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1V3nne-0007D2-Up
	for qemu-devel@nongnu.org; Mon, 29 Jul 2013 09:47:53 -0400
Received: from mx1.redhat.com ([209.132.183.28]:56455)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1V3nne-0007Ct-OE
	for qemu-devel@nongnu.org; Mon, 29 Jul 2013 09:47:46 -0400
Message-ID: <51F6726D.5020405@redhat.com>
Date: Mon, 29 Jul 2013 15:47:25 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
MIME-Version: 1.0
References: <1375100921-12990-1-git-send-email-pbonzini@redhat.com>
	<CAFEAcA_5M0Vyw=ydbj4FdBTgTZu0bw+zBOJutTBsz=JWtMeY9A@mail.gmail.com>
In-Reply-To: <CAFEAcA_5M0Vyw=ydbj4FdBTgTZu0bw+zBOJutTBsz=JWtMeY9A@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH for-1.6] exec: fix writing to MMIO area
 with non-power-of-two length
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 Maydell <peter.maydell@linaro.org>
Cc: alxchk@gmail.com, qemu-devel@nongnu.org

Il 29/07/2013 14:41, Peter Maydell ha scritto:
> On 29 July 2013 13:28, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> The problem is introduced by commit 2332616 (exec: Support 64-bit
>> operations in address_space_rw, 2013-07-08).  Before that commit,
>> memory_access_size would only return 1/2/4.
>>
>> Since alignment is already handled above, reduce l to the largest
>> power of two that is smaller than l.
>>
>> Reported-by: Oleksii Shevchuk <alxchk@gmail.com>
>> Tested-by: Oleksii Shevchuk <alxchk@gmail.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>  exec.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/exec.c b/exec.c
>> index c4f2894..e122c81 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -1925,6 +1925,9 @@ static int memory_access_size(MemoryRegion *mr, unsigned l, hwaddr addr)
>>      if (l > access_size_max) {
>>          l = access_size_max;
>>      }
>> +    if (l & (l - 1)) {
>> +        l = 1 << (qemu_fls(l) - 1);
>> +    }
> 
> Is this a hot enough code path that we care about going via
> the not-inline qemu_fls() rather than calling clz32() directly?

It is not that hot because of the "if".

> (probably not, I guess.) Alternatively, we seem to have a
> pow2floor() function...

Yeah, pow2floor is also nice.  There's still a lot of opportunity for
unification...

Paolo