linux-kernel - Re: [PATCH] riscv: pass machine size to sparse

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Message-ID: <mhng-a4f5b769-8271-4b99-ae61-bbc49fa2286f@palmer-si-x1c4>
Date:   Thu, 31 May 2018 05:09:21 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     yamada.masahiro@...ionext.com, luc.vanoostenryck@...il.com
CC:     Christoph Hellwig <hch@...radead.org>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        albert@...ive.com, michal.lkml@...kovi.net
Subject:     Re: [PATCH] riscv: pass machine size to sparse

On Mon, 28 May 2018 23:14:20 PDT (-0700), yamada.masahiro@...ionext.com wrote:
> 2018-05-29 15:11 GMT+09:00 Christoph Hellwig <hch@...radead.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.  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, 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.  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.

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.

Thanks!