linux-kernel - Re: [PATCH] riscv: Fix ALT_THEAD

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Message-ID: <CAKwvOdk3HZkiZwtp1+xZULbm8BkOjdCQxPqsOHDJ5CboVy5ONw@mail.gmail.com>
Date:   Wed, 18 May 2022 13:45:15 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Heiko Stuebner <heiko@...ech.de>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, patches@...ts.linux.dev,
        Jessica Clarke <jrtc27@...c27.com>
Subject: Re: [PATCH] riscv: Fix ALT_THEAD_PMA's asm parameters

On Wed, May 18, 2022 at 11:45 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> After commit a35707c3d850 ("riscv: add memory-type errata for T-Head"),
> builds with LLVM's integrated assembler fail like:
>
>   In file included from arch/riscv/kernel/asm-offsets.c:10:
>   In file included from ./include/linux/mm.h:29:
>   In file included from ./include/linux/pgtable.h:6:
>   In file included from ./arch/riscv/include/asm/pgtable.h:114:
>   ./arch/riscv/include/asm/pgtable-64.h:210:2: error: invalid input constraint '0' in asm
>           ALT_THEAD_PMA(prot_val);
>           ^
>   ./arch/riscv/include/asm/errata_list.h:88:4: note: expanded from macro 'ALT_THEAD_PMA'
>           : "0"(_val),                                                    \
>             ^
>
> This was reported upstream to LLVM where Jessica pointed out a couple of
> issues with the existing implementation of ALT_THEAD_PMA:
>
> * t3 is modified but not listed in the clobbers list.
>
> * "+r"(_val) marks _val as both an input and output of the asm but then
>   "0"(_val) marks _val as an input matching constraint, which does not
>   make much sense in this situation, as %1 is not actually used in the
>   asm and matching constraints are designed to be used for different
>   inputs that need to use the same register.
>
> Drop the matching contraint and shift all the operands by one, as %1 is

s/contraint/constraint/

Thanks for the fix!
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

> unused, and mark t3 as clobbered. This resolves the build error and goes
> not cause any problems with GNU as.
>
> Fixes: a35707c3d850 ("riscv: add memory-type errata for T-Head")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1641
> Link: https://github.com/llvm/llvm-project/issues/55514
> Link: https://gcc.gnu.org/onlinedocs/gcc/Simple-Constraints.html
> Suggested-by: Jessica Clarke <jrtc27@...c27.com>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> ---
>  arch/riscv/include/asm/errata_list.h | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
> index 9e2888dbb5b1..416ead0f9a65 100644
> --- a/arch/riscv/include/asm/errata_list.h
> +++ b/arch/riscv/include/asm/errata_list.h
> @@ -75,20 +75,20 @@ asm volatile(ALTERNATIVE(                                           \
>         "nop\n\t"                                                       \
>         "nop\n\t"                                                       \
>         "nop",                                                          \
> -       "li      t3, %2\n\t"                                            \
> -       "slli    t3, t3, %4\n\t"                                        \
> +       "li      t3, %1\n\t"                                            \
> +       "slli    t3, t3, %3\n\t"                                        \
>         "and     t3, %0, t3\n\t"                                        \
>         "bne     t3, zero, 2f\n\t"                                      \
> -       "li      t3, %3\n\t"                                            \
> -       "slli    t3, t3, %4\n\t"                                        \
> +       "li      t3, %2\n\t"                                            \
> +       "slli    t3, t3, %3\n\t"                                        \
>         "or      %0, %0, t3\n\t"                                        \
>         "2:",  THEAD_VENDOR_ID,                                         \
>                 ERRATA_THEAD_PBMT, CONFIG_ERRATA_THEAD_PBMT)            \
>         : "+r"(_val)                                                    \
> -       : "0"(_val),                                                    \
> -         "I"(_PAGE_MTMASK_THEAD >> ALT_THEAD_PBMT_SHIFT),              \
> +       : "I"(_PAGE_MTMASK_THEAD >> ALT_THEAD_PBMT_SHIFT),              \
>           "I"(_PAGE_PMA_THEAD >> ALT_THEAD_PBMT_SHIFT),                 \
> -         "I"(ALT_THEAD_PBMT_SHIFT))
> +         "I"(ALT_THEAD_PBMT_SHIFT)                                     \
> +       : "t3")
>  #else
>  #define ALT_THEAD_PMA(_val)
>  #endif
>
> base-commit: 93c0651617a62a69717299f1464dda798af8bebb
> --
> 2.36.1
>


-- 
Thanks,
~Nick Desaulniers