From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <dev-bounces@dpdk.org>
Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124])
	by inbox.dpdk.org (Postfix) with ESMTP id C944043DE9;
	Tue,  9 Apr 2024 10:28:52 +0200 (CEST)
Received: from mails.dpdk.org (localhost [127.0.0.1])
	by mails.dpdk.org (Postfix) with ESMTP id 9EF6B402C0;
	Tue,  9 Apr 2024 10:28:52 +0200 (CEST)
Received: from frasgout.his.huawei.com (frasgout.his.huawei.com
 [185.176.79.56])
 by mails.dpdk.org (Postfix) with ESMTP id DAEBF402B9;
 Tue,  9 Apr 2024 10:28:50 +0200 (CEST)
Received: from mail.maildlp.com (unknown [172.18.186.231])
 by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4VDJry5KgYz6K92N;
 Tue,  9 Apr 2024 16:24:02 +0800 (CST)
Received: from frapeml100008.china.huawei.com (unknown [7.182.85.131])
 by mail.maildlp.com (Postfix) with ESMTPS id 5B8211400CA;
 Tue,  9 Apr 2024 16:28:48 +0800 (CST)
Received: from frapeml500007.china.huawei.com (7.182.85.172) by
 frapeml100008.china.huawei.com (7.182.85.131) with Microsoft SMTP Server
 (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id
 15.1.2507.35; Tue, 9 Apr 2024 10:28:48 +0200
Received: from frapeml500007.china.huawei.com ([7.182.85.172]) by
 frapeml500007.china.huawei.com ([7.182.85.172]) with mapi id 15.01.2507.035;
 Tue, 9 Apr 2024 10:28:48 +0200
From: Konstantin Ananyev <konstantin.ananyev@huawei.com>
To: =?iso-8859-1?Q?Morten_Br=F8rup?= <mb@smartsharesystems.com>, "Tyler
 Retzlaff" <roretzla@linux.microsoft.com>, Stephen Hemminger
 <stephen@networkplumber.org>, "techboard@dpdk.org" <techboard@dpdk.org>
CC: =?iso-8859-1?Q?Mattias_R=F6nnblom?= <hofors@lysator.liu.se>,
 "dev@dpdk.org" <dev@dpdk.org>, Bruce Richardson <bruce.richardson@intel.com>, 
 Thomas Monjalon <thomas@monjalon.net>
Subject: RE: [PATCH 0/4] RFC samples converting VLA to alloca
Thread-Topic: [PATCH 0/4] RFC samples converting VLA to alloca
Thread-Index: AQHahrO8mqsV6ha1EkmL480J0CFr3rFcbeUAgAAaoACAAGN3AIABd3+AgAAHW4CAATbNUA==
Date: Tue, 9 Apr 2024 08:28:48 +0000
Message-ID: <09aac29da90a499ebfc16493e9942bf6@huawei.com>
References: <20231107193220.GA15232@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
 <1712250913-1977-1-git-send-email-roretzla@linux.microsoft.com>
 <adb81b30-bece-416b-a233-b08873d196f6@lysator.liu.se>
 <98CBD80474FA8B44BF855DF32C47DC35E9F379@smartserver.smartshare.dk>
 <20240407100306.36c9688f@hermes.local>
 <20240408152703.GA25804@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
 <98CBD80474FA8B44BF855DF32C47DC35E9F380@smartserver.smartshare.dk>
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35E9F380@smartserver.smartshare.dk>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach: 
X-MS-TNEF-Correlator: 
x-originating-ip: [10.206.138.42]
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-BeenThere: dev@dpdk.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: DPDK patches and discussions <dev.dpdk.org>
List-Unsubscribe: <https://mails.dpdk.org/options/dev>,
 <mailto:dev-request@dpdk.org?subject=unsubscribe>
List-Archive: <http://mails.dpdk.org/archives/dev/>
List-Post: <mailto:dev@dpdk.org>
List-Help: <mailto:dev-request@dpdk.org?subject=help>
List-Subscribe: <https://mails.dpdk.org/listinfo/dev>,
 <mailto:dev-request@dpdk.org?subject=subscribe>
Errors-To: dev-bounces@dpdk.org



> > From: Tyler Retzlaff [mailto:roretzla@linux.microsoft.com]
> > Sent: Monday, 8 April 2024 17.27
> >
> > For next technboard meeting.
> >
> > On Sun, Apr 07, 2024 at 10:03:06AM -0700, Stephen Hemminger wrote:
> > > On Sun, 7 Apr 2024 13:07:06 +0200
> > > Morten Br=F8rup <mb@smartsharesystems.com> wrote:
> > >
> > > > > From: Mattias R=F6nnblom [mailto:hofors@lysator.liu.se]
> > > > > Sent: Sunday, 7 April 2024 11.32
> > > > >
> > > > > On 2024-04-04 19:15, Tyler Retzlaff wrote:
> > > > > > This series is not intended for merge.  It insteat provides exa=
mples
> > > > > of
> > > > > > converting use of VLAs to alloca() would look like.
> > > > > >
> > > > > > what's the advantages of VLA over alloca()?
> > > > > >
> > > > > > * sizeof(array) works as expected.
> > > > > >
> > > > > > * multi-dimensional arrays are still arrays instead of pointers=
 to
> > > > > >    dynamically allocated space. this means multiple subscript s=
yntax
> > > > > >    works (unlike on a pointer) and calculation of addresses int=
o
> > > > > allocated
> > > > > >    space in ascending order is performed by the compiler instea=
d of
> > > > > manually.
> > > > > >
> > > > >
> > > > > alloca() is a pretty obscure mechanism, and also not a part of th=
e C
> > > > > standard. VLAs are C99, and well-known and understood, and very
> > > > > efficient.
> > > >
> > > > The RFC fails to mention why we need to replace VLAs with something=
 else:
> > > >
> > > > VLAs are C99, but not C++; VLAs were made optional in C11.
> > > >
> > > > MSVC doesn't support VLAs, and is not going to:
> > > > https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-support=
-
> > arriving-in-msvc/#variable-length-arrays
> > > >
> > > >
> > > > I dislike alloca() too, and the notes section in the alloca(3) man =
page
> > even discourages the use of alloca():
> > > > https://man7.org/linux/man-pages/man3/alloca.3.html
> > > >
> > > > But I guess alloca() is the simplest replacement for VLAs.
> > > > This RFC patch series opens the discussion for alternatives in diff=
erent
> > use cases.
> > > >
> > >
> > > The other issue with VLA's is that if the number is something that ca=
n be
> > externally
> > > input, then it can be a source of stack overflow bugs. That is why th=
e Linux
> > kernel
> > > has stopped using them; for security reasons. DPDK has much less of a
> > security
> > > trust domain. Mostly need to make sure that no data from network is b=
eing
> > > used to compute VLA size.
> > >
> >
> > Looks like we need to discuss this at the next techboard meeting.
> >
> > * MSVC doesn't support C11 optional VLAs (and never will).
> > * alloca() is an alternative that is available on all platforms/toolcha=
in
> >   combinations.
> > * it's reasonable for some VLAs to be turned into regular arrays but it
> >   would be unsatisfactory to be stuck waiting discussions of defining n=
ew
> >   constant expression macros on a per-use basis.
>=20
> We must generally stop using VLAs, for many reasons.
> The only available 1:1 replacement is alloca(), so we have to accept that=
.
>=20
> If anyone still cares about improvements, we can turn alloca()'d arrays i=
nto regular arrays after this patch series.
>=20
> Alternatives to VLAs are very interesting discussions, but let's not stal=
l MSVC progress because of it!

Ok, but why we have to rush into 'alloca()' solution if none of us really f=
ond of it?
As you already noted majority of these cases can be replaced with static si=
zed arrays.
Let's try to compile a list of what needs to be changed, split it by priori=
ties and work
progressively through it.=20
Konstantin=20

>=20
> > * there is resistance to using alloca() vs VLA so my proposal is to
> >   change only the code that is built to target windows.
>=20
> I would prefer to get rid of them all, so the CI can build with -Wvla to =
prevent them from being introduced again.
> Not a strong preference.
> On the other hand, the CI's MSVC builds will catch them if used for a Win=
dows target.
> And limiting to Windows code reduces the amount of work, so that's probab=
ly the most realistic solution.