linux-kernel - Re: [RFC PATCH 01/11] efi: Fix gcc error around _

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Date:   Fri, 26 Jun 2020 14:56:11 -0700
From:   Atish Patra <atishp@...shpatra.org>
To:     Heinrich Schuchardt <xypron.glpk@....de>
Cc:     Atish Patra <atish.patra@....com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 01/11] efi: Fix gcc error around __umoddi3 for 32 bit builds

On Thu, Jun 25, 2020 at 7:43 PM Heinrich Schuchardt <xypron.glpk@....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@....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.
> Best regards
>
> Heinrich
>
> >
> >               if (l) {
> >                       efi_bs_call(free_pages, alloc_addr, slack - l + 1);
> >
>


-- 
Regards,
Atish