Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 15 Aug 2019 11:12:25 +0800
From:   Nick Hu <nickhu@andestech.com>
To:     Paul Walmsley <paul.walmsley@sifive.com>
CC:     Palmer Dabbelt <palmer@sifive.com>,
        Christoph Hellwig <hch@infradead.org>,
        Alan Quey-Liang =?utf-8?B?S2FvKOmrmOmtgeiJryk=?= 
        <alankao@andestech.com>,
        "aou@eecs.berkeley.edu" <aou@eecs.berkeley.edu>,
        "green.hu@gmail.com" <green.hu@gmail.com>,
        "deanbo422@gmail.com" <deanbo422@gmail.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "linux-riscv@lists.infradead.org" <linux-riscv@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "aryabinin@virtuozzo.com" <aryabinin@virtuozzo.com>,
        "glider@google.com" <glider@google.com>,
        "dvyukov@google.com" <dvyukov@google.com>,
        Anup Patel <Anup.Patel@wdc.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        "alexios.zavras@intel.com" <alexios.zavras@intel.com>,
        Atish Patra <Atish.Patra@wdc.com>,
        =?utf-8?B?6Zui6IG3Wm9uZyBab25nLVhpYW4gTGko5p2O5a6X5oayKQ==?= 
        <zong@andestech.com>,
        "kasan-dev@googlegroups.com" <kasan-dev@googlegroups.com>
Subject: Re: [PATCH 1/2] riscv: Add memmove string operation.
Message-ID: <20190815031225.GA5666@andestech.com>
References: <mhng-ba92c635-7087-4783-baa5-2a111e0e2710@palmer-si-x1e>
 <alpine.DEB.2.21.9999.1908131921180.19217@viisi.sifive.com>
 <20190814032732.GA8989@andestech.com>
 <alpine.DEB.2.21.9999.1908141002500.18249@viisi.sifive.com>
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Disposition: inline
In-Reply-To: <alpine.DEB.2.21.9999.1908141002500.18249@viisi.sifive.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi Paul,

On Wed, Aug 14, 2019 at 10:03:39AM -0700, Paul Walmsley wrote:
> Hi Nick,
> 
> On Wed, 14 Aug 2019, Nick Hu wrote:
> 
> > On Wed, Aug 14, 2019 at 10:22:15AM +0800, Paul Walmsley wrote:
> > > On Tue, 13 Aug 2019, Palmer Dabbelt wrote:
> > > 
> > > > On Mon, 12 Aug 2019 08:04:46 PDT (-0700), Christoph Hellwig wrote:
> > > > > On Wed, Aug 07, 2019 at 03:19:14PM +0800, Nick Hu wrote:
> > > > > > There are some features which need this string operation for compilation,
> > > > > > like KASAN. So the purpose of this porting is for the features like KASAN
> > > > > > which cannot be compiled without it.
> > > > > > 
> > > > > > KASAN's string operations would replace the original string operations and
> > > > > > call for the architecture defined string operations. Since we don't have
> > > > > > this in current kernel, this patch provides the implementation.
> > > > > > 
> > > > > > This porting refers to the 'arch/nds32/lib/memmove.S'.
> > > > > 
> > > > > This looks sensible to me, although my stringop asm is rather rusty,
> > > > > so just an ack and not a real review-by:
> > > > 
> > > > FWIW, we just write this in C everywhere else and rely on the compiler to
> > > > unroll the loops.  I always prefer C to assembly when possible, so I'd prefer
> > > > if we just adopt the string code from newlib.  We have a RISC-V-specific
> > > > memcpy in there, but just use the generic memmove.
> > > > 
> > > > Maybe the best bet here would be to adopt the newlib memcpy/memmove as generic
> > > > Linux functions?  They're both in C so they should be fine, and they both look
> > > > faster than what's in lib/string.c.  Then everyone would benefit and we don't
> > > > need this tricky RISC-V assembly.  Also, from the look of it the newlib code
> > > > is faster because the inner loop is unrolled.
> > > 
> > > There's a generic memmove implementation in the kernel already:
> > > 
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/string.h#n362
> > > 
> > > Nick, could you tell us more about why the generic memmove() isn't 
> > > suitable?
> > 
> > KASAN has its own string operations(memcpy/memmove/memset) because it needs to
> > hook some code to check memory region. It would undefined the original string
> > operations and called the string operations with the prefix '__'. But the
> > generic string operations didn't declare with the prefix. Other archs with
> > KASAN support like arm64 and xtensa all have their own string operations and
> > defined with the prefix.
> 
> Thanks for the explanation.  What do you think about Palmer's idea to 
> define a generic C set of KASAN string operations, derived from the newlib 
> code?
> 
> 
> - Paul

That sounds good to me. But it should be another topic. We need to investigate
it further about replacing something generic and fundamental in lib/string.c
with newlib C functions.  Some blind spots may exist.  So I suggest, let's
consider KASAN for now.

Nick