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Date:   Mon, 04 Jun 2018 18:18:50 -0700 (PDT)
Subject:     Re: [PATCH] riscv: pass machine size to sparse
In-Reply-To: <20180531153553.kex32gupopdq2nqh@ltop.local>
CC:     yamada.masahiro@socionext.com,
        Christoph Hellwig <hch@infradead.org>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        albert@sifive.com, michal.lkml@markovi.net
From:   Palmer Dabbelt <palmer@sifive.com>
To:     luc.vanoostenryck@gmail.com
Message-ID: <mhng-fc62e953-d986-4975-9164-0eafc1ade1d3@palmer-si-x1c4>
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On Thu, 31 May 2018 08:35:54 PDT (-0700), luc.vanoostenryck@gmail.com wrote:
> On Thu, May 31, 2018 at 05:09:21AM -0700, Palmer Dabbelt wrote:
>> On Mon, 28 May 2018 23:14:20 PDT (-0700), yamada.masahiro@socionext.com wrote:
>> > 2018-05-29 15:11 GMT+09:00 Christoph Hellwig <hch@infradead.org>:
>> > > On Mon, May 28, 2018 at 06:35:05PM +0200, Luc Van Oostenryck wrote:
>> > > > By default, sparse assumes a 64bit machine when compiled on x86-64
>> > > > and 32bit when compiled on anything else.
>> > > >
>> > > > This can of course create all sort of problems when this doesn't
>> > > > correspond to the target's machine size, like issuing false
>> > > > warnings like: 'shift too big (32) for type unsigned long' or
>> > > > is 64bit while sparse was compiled on a 32bit machine, or worse,
>> > > > to not emit legitimate warnings.
>> > > >
>> > > > Fix this by passing the appropriate -m32/-m64 flag to sparse.
>> > >
>> > > Can we please move this to the common Kbuild code using the
>> > > CONFIG_64BIT syombol?  This really should not need boiler plate in
>> > > every architecture.
>> >
>> >
>> > I agree.
>> >
>> > Luc did so for -mbig/little-endian:
>> > https://patchwork.kernel.org/patch/10433957/
>> >
>> > We should do likewise for -m32/64.
>>
>> Sorry for being a bit slow here, but I like the idea of making the
>> 32/64-bit issue generic as it seems like it'll be necessary for
>> every port.
>
> Sure, Christophe asked it too.
> I've sent a new version doing it once and for all for all archs
> but I forgot to CC you:
> 	https://lkml.org/lkml/2018/5/30/948
>
>> Looking through the patch for big/little-endian I did
>> notice:
>>
>> * RISC-V compilers set "__riscv_xlen" to the length of an X
>> (integer) register  in bits.
>> * RISC-V compilers define "__riscv", and it doesn't appear we inform
>> sparse  about that.
>>
>> These two might not be that interesting, but we do already have some
>> cases where we're checking for __riscv_xlen in Linux.  I've yet to
>> successfully use sparse, but adding at least
>>
>>    CHECKFLAGS += -D__riscv
>>
>> seems reasonable,
>
> Sure (but I don't see a dependency in the kernel (yet)).
>
>> and possibly also some sort of
>>
>>    ifeq ($(CONFIG_ARCH_RV64I),y)
>>            CHECKFLAGS += -D__riscv_xlen=64
>>    else
>>            CHECKFLAGS += -D__riscv_xlen=32
>>    fi
>>
>> might be necessary.
>
> Yes, this one is really needed.
> I'll send a patch for this one and __riscv.
>
>> We strive to follow the generic rules for
>> ABI-related stuff like __LP64__ but I don't think there's any
>> generic mapping for XLEN.  Similarly there's "__riscv_flen" and
>> "__riscv_float_abi_*", but those are less likely to be used by the
>> kernel so they're probably not worth worrying about for now.
>
> Yes, I agree.
> Note that sparse will define __LP64__ (and _LP64) when in -m64 mode.
>
>> There's also a bunch of other RISC-V macros, the only one of which
>> we're currently using is "__riscv_muldiv" (and that's in a manner
>> that's unlikely to trigger any sort of static analysis).  Between a
>> lack of Kconfig options and a glibc port we're essentially mandating
>> IMA right now, so these probably don't matter.
>
> Yes, I just saw. I think also it's better to leave it so for now.
> And if it becomes more used, then better to infer it from the compiler
> than harcoding it.

Makes sense.  Thanks for the work!