Re: [PATCH v4 04/18] arm64: Do not enable uaccess for flush_icache_range

[Ard Biesheuvel <ardb@kernel.org>]

On Mon, 24 May 2021 at 10:30, Fuad Tabba <tabba@google.com> wrote:
>
> __flush_icache_range works on kernel addresses, and doesn't need
> uaccess. The existing code is a side-effect of its current
> implementation with __flush_cache_user_range fallthrough.
>
> Instead of fallthrough to share the code, use a common macro for
> the two where the caller specifies an optional fixup label if
> user access is needed. If provided, this label would be used to
> generate an extable entry.
>
> Simplify the code to use dcache_by_line_op, instead of
> replicating much of its functionality.
>
> No functional change intended.
> Possible performance impact due to the reduced number of
> instructions.
>
> Reported-by: Catalin Marinas <catalin.marinas@arm.com>
> Reported-by: Will Deacon <will@kernel.org>
> Reported-by: Mark Rutland <mark.rutland@arm.com>
> Link: https://lore.kernel.org/linux-arch/20200511110014.lb9PEahJ4hVOYrbwIb_qUHXyNy9KQzNFdb_I3YlzY6A@z/
> Link: https://lore.kernel.org/linux-arm-kernel/20210521121846.GB1040@C02TD0UTHF1T.local/
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  arch/arm64/mm/cache.S | 57 ++++++++++++++++++++++++++-----------------
>  1 file changed, 34 insertions(+), 23 deletions(-)
>
> diff --git a/arch/arm64/mm/cache.S b/arch/arm64/mm/cache.S
> index 2d881f34dd9d..7c54bcbf5a36 100644
> --- a/arch/arm64/mm/cache.S
> +++ b/arch/arm64/mm/cache.S
> @@ -14,6 +14,34 @@
>  #include <asm/alternative.h>
>  #include <asm/asm-uaccess.h>
>
> +/*
> + *     __flush_cache_range(start,end) [fixup]
> + *
> + *     Ensure that the I and D caches are coherent within specified region.
> + *     This is typically used when code has been written to a memory region,
> + *     and will be executed.
> + *
> + *     - start   - virtual start address of region
> + *     - end     - virtual end address of region
> + *     - fixup   - optional label to branch to on user fault
> + */
> +.macro __flush_cache_range, fixup
> +alternative_if ARM64_HAS_CACHE_IDC
> +       dsb     ishst

Should this perhaps be dsb ish? IIUC, ishst does not synchronize on
completion of cache maintenance, and while that is implicit on this
code path, I'd still assume it needs to complete before carrying on.
Or does IDC not require this?

> +       b       .Ldc_skip_\@
> +alternative_else_nop_endif
> +       mov     x2, x0
> +       sub     x3, x1, x0
> +       dcache_by_line_op cvau, ish, x2, x3, x4, x5, \fixup
> +.Ldc_skip_\@:
> +alternative_if ARM64_HAS_CACHE_DIC
> +       isb
> +       b       .Lic_skip_\@
> +alternative_else_nop_endif
> +       invalidate_icache_by_line x0, x1, x2, x3, \fixup
> +.Lic_skip_\@:
> +.endm
> +
>  /*
>   *     flush_icache_range(start,end)
>   *
> @@ -25,7 +53,9 @@
>   *     - end     - virtual end address of region
>   */
>  SYM_FUNC_START(__flush_icache_range)
> -       /* FALLTHROUGH */
> +       __flush_cache_range
> +       ret
> +SYM_FUNC_END(__flush_icache_range)
>
>  /*
>   *     __flush_cache_user_range(start,end)
> @@ -39,34 +69,15 @@ SYM_FUNC_START(__flush_icache_range)
>   */
>  SYM_FUNC_START(__flush_cache_user_range)
>         uaccess_ttbr0_enable x2, x3, x4
> -alternative_if ARM64_HAS_CACHE_IDC
> -       dsb     ishst
> -       b       7f
> -alternative_else_nop_endif
> -       dcache_line_size x2, x3
> -       sub     x3, x2, #1
> -       bic     x4, x0, x3
> -1:
> -user_alt 9f, "dc cvau, x4",  "dc civac, x4",  ARM64_WORKAROUND_CLEAN_CACHE
> -       add     x4, x4, x2
> -       cmp     x4, x1
> -       b.lo    1b
> -       dsb     ish
>
> -7:
> -alternative_if ARM64_HAS_CACHE_DIC
> -       isb
> -       b       8f
> -alternative_else_nop_endif
> -       invalidate_icache_by_line x0, x1, x2, x3, 9f
> -8:     mov     x0, #0
> +       __flush_cache_range 2f
> +       mov     x0, xzr
>  1:
>         uaccess_ttbr0_disable x1, x2
>         ret
> -9:
> +2:
>         mov     x0, #-EFAULT
>         b       1b
> -SYM_FUNC_END(__flush_icache_range)
>  SYM_FUNC_END(__flush_cache_user_range)
>
>  /*
> --
> 2.31.1.818.g46aad6cb9e-goog
>

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