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Date: Thu, 1 Feb 2024 18:32:50 -0800
From: Charlie Jenkins <charlie@rivosinc.com>
To: =?iso-8859-1?Q?Cl=E9ment_L=E9ger?= <cleger@rivosinc.com>
Cc: Jonathan Corbet <corbet@lwn.net>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>, linux-doc@vger.kernel.org,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	Robin Ehn <rehn@rivosinc.com>
Subject: Re: [PATCH] riscv: hwprobe: export VA_BITS
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References: <20240201140319.360088-1-cleger@rivosinc.com>
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On Thu, Feb 01, 2024 at 03:02:45PM +0100, Cl?ment L?ger wrote:
> Some userspace applications (OpenJDK for instance) uses the free bits
> in pointers to insert additional information for their own logic.
> Currently they rely on parsing /proc/cpuinfo to obtain the current value
> of virtual address used bits [1]. Exporting VA_BITS through hwprobe will
> allow for a more stable interface to be used.

mmap already supports this without a need for applications to know the
underlying hardware. If a hint address is passed into mmap, it will never
return an address that uses more bits than the hint address. I designed
it that way so that something like this wasn't necessary.

- Charlie

> 
> Link: https://github.com/openjdk/jdk/blob/master/src/hotspot/os_cpu/linux_riscv/vm_version_linux_riscv.cpp#L171 [1]
> Signed-off-by: Cl?ment L?ger <cleger@rivosinc.com>
> 
> ---
>  Documentation/arch/riscv/hwprobe.rst  | 3 +++
>  arch/riscv/include/asm/hwprobe.h      | 2 +-
>  arch/riscv/include/uapi/asm/hwprobe.h | 1 +
>  arch/riscv/kernel/sys_hwprobe.c       | 3 +++
>  4 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
> index b2bcc9eed9aa..6f198c6ed4f0 100644
> --- a/Documentation/arch/riscv/hwprobe.rst
> +++ b/Documentation/arch/riscv/hwprobe.rst
> @@ -210,3 +210,6 @@ The following keys are defined:
>  
>  * :c:macro:`RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE`: An unsigned int which
>    represents the size of the Zicboz block in bytes.
> +
> +* :c:macro:`RISCV_HWPROBE_KEY_VA_BITS`: An unsigned long which
> +  represent the number of bits used to store virtual addresses.
> diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
> index 630507dff5ea..150a9877b0af 100644
> --- a/arch/riscv/include/asm/hwprobe.h
> +++ b/arch/riscv/include/asm/hwprobe.h
> @@ -8,7 +8,7 @@
>  
>  #include <uapi/asm/hwprobe.h>
>  
> -#define RISCV_HWPROBE_MAX_KEY 6
> +#define RISCV_HWPROBE_MAX_KEY 7
>  
>  static inline bool riscv_hwprobe_key_is_valid(__s64 key)
>  {
> diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
> index 9f2a8e3ff204..2a5006cddb7b 100644
> --- a/arch/riscv/include/uapi/asm/hwprobe.h
> +++ b/arch/riscv/include/uapi/asm/hwprobe.h
> @@ -67,6 +67,7 @@ struct riscv_hwprobe {
>  #define		RISCV_HWPROBE_MISALIGNED_UNSUPPORTED	(4 << 0)
>  #define		RISCV_HWPROBE_MISALIGNED_MASK		(7 << 0)
>  #define RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE	6
> +#define RISCV_HWPROBE_KEY_VA_BITS		7
>  /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
>  
>  /* Flags */
> diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
> index a7c56b41efd2..328435836e36 100644
> --- a/arch/riscv/kernel/sys_hwprobe.c
> +++ b/arch/riscv/kernel/sys_hwprobe.c
> @@ -202,6 +202,9 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
>  		if (hwprobe_ext0_has(cpus, RISCV_HWPROBE_EXT_ZICBOZ))
>  			pair->value = riscv_cboz_block_size;
>  		break;
> +	case RISCV_HWPROBE_KEY_VA_BITS:
> +		pair->value = VA_BITS;
> +		break;
>  
>  	/*
>  	 * For forward compatibility, unknown keys don't fail the whole
> -- 
> 2.43.0
> 
> 
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