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Date:   Mon, 17 Jun 2019 18:32:41 +0100
From:   Will Deacon <will.deacon@arm.com>
To:     Florian Fainelli <f.fainelli@gmail.com>
Cc:     linux-arm-kernel@vger.kernel.org,
        bcm-kernel-feedback-list@Broadcom.com, ard.biesheuvel@linaro.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" 
        <linux-arm-kernel@lists.infradead.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] arm64: Allow user selection of ARM64_MODULE_PLTS
Message-ID: <20190617173241.GM30800@fuggles.cambridge.arm.com>
References: <20190614025932.533-1-f.fainelli@gmail.com>
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On Thu, Jun 13, 2019 at 07:59:32PM -0700, Florian Fainelli wrote:
> Make ARM64_MODULE_PLTS a selectable Kconfig symbol, since some people
> might have very big modules spilling out of the dedicated module area
> into vmalloc. Help text is copied from the ARM 32-bit counterpart.
> 
> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
> ---
>  arch/arm64/Kconfig | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 697ea0510729..36befe987b73 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1418,8 +1418,20 @@ config ARM64_SVE
>  	  KVM in the same kernel image.
>  
>  config ARM64_MODULE_PLTS
> -	bool
> +	bool "Use PLTs to allow module memory to spill over into vmalloc area"
>  	select HAVE_MOD_ARCH_SPECIFIC
> +	help
> +	  Allocate PLTs when loading modules so that jumps and calls whose
> +	  targets are too far away for their relative offsets to be encoded
> +	  in the instructions themselves can be bounced via veneers in the
> +	  module's PLT. This allows modules to be allocated in the generic
> +	  vmalloc area after the dedicated module memory area has been
> +	  exhausted. The modules will use slightly more memory, but after
> +	  rounding up to page size, the actual memory footprint is usually
> +	  the same.

Isn't the worry really about the runtime performance overhead introduced
by the veneers, as opposed to the memory usage of the module?

> +	  Disabling this is usually safe for small single-platform
> +	  configurations. If unsure, say y.

So should this be on by default?

Will