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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Sat, 27 Jun 2020 00:03:46 +0200
Message-ID: <CAMj1kXFyQiYPYB-81POq7agRW1ROt=2j3nN9wcpHGmr4YjmFCQ@mail.gmail.com>
Subject: Re: [RFC PATCH 01/11] efi: Fix gcc error around __umoddi3 for 32 bit builds
To:     Atish Patra <atishp@atishpatra.org>
Cc:     Heinrich Schuchardt <xypron.glpk@gmx.de>,
        Atish Patra <atish.patra@wdc.com>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>,
        linux-efi <linux-efi@vger.kernel.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>
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On Fri, 26 Jun 2020 at 23:56, Atish Patra <atishp@atishpatra.org> wrote:
>
> On Thu, Jun 25, 2020 at 7:43 PM Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
> >
> > On 6/26/20 1:45 AM, Atish Patra wrote:
> > > 32bit gcc doesn't support modulo operation on 64 bit data. It results in
> > > a __umoddi3 error while building EFI for 32 bit.
> > >
> > > Use bitwise operations instead of modulo operations to fix the issue.
> > >
> > > Signed-off-by: Atish Patra <atish.patra@wdc.com>
> > > ---
> > >  drivers/firmware/efi/libstub/alignedmem.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/firmware/efi/libstub/alignedmem.c b/drivers/firmware/efi/libstub/alignedmem.c
> > > index cc89c4d6196f..1de9878ddd3a 100644
> > > --- a/drivers/firmware/efi/libstub/alignedmem.c
> > > +++ b/drivers/firmware/efi/libstub/alignedmem.c
> > > @@ -44,7 +44,7 @@ efi_status_t efi_allocate_pages_aligned(unsigned long size, unsigned long *addr,
> > >       *addr = ALIGN((unsigned long)alloc_addr, align);
> > >
> > >       if (slack > 0) {
> > > -             int l = (alloc_addr % align) / EFI_PAGE_SIZE;
> > > +             int l = (alloc_addr & (align - 1)) / EFI_PAGE_SIZE;
> >
> > Here you rely on the compiler to silently convert the division by
> > EFI_PAGE_SIZE into a right shift. Wouldn't it be cleaner to use
> > EFI_PAGE_SHIFT to explicitly avoid a dependency on __udivdi3()?
> >
> > slack = (align >> EFI_PAGE_SHIFT) - 1;
> > ...
> > int l = (alloc_addr & (align - 1)) >> EFI_PAGE_SHIFT;
> >
>
> Sure. I will update it in the next version. Thanks for the suggestion.

Please don't. Dividing by EFI_PAGE_SIZE is perfectly fine, and is more readable.