Re: [PATCH] riscv: hwprobe: export highest virtual userspace address

[Charlie Jenkins]

On Fri, Mar 01, 2024 at 05:44:06PM +0100, Clément Léger wrote:
> Some userspace applications (OpenJDK for instance) uses the free MSBs
> in pointers to insert additional information for their own logic and
> need to get this information from somewhere. Currently they rely on
> parsing /proc/cpuinfo "mmu=svxx" string to obtain the current value of
> virtual address usable bits [1]. Since this reflect the raw supported
> MMU mode, it might differ from the logical one used internally which is
> why arch_get_mmap_end() is used. Exporting the highest mmapable address
> through hwprobe will allow a more stable interface to be used. For that
> purpose, add a new hwprobe key named
> RISCV_HWPROBE_KEY_HIGHEST_VIRT_ADDRESS which will export the highest
> userspace virtual address.
> 
> Link: https://github.com/openjdk/jdk/blob/master/src/hotspot/os_cpu/linux_riscv/vm_version_linux_riscv.cpp#L171 [1]
> Signed-off-by: Clément Léger <cleger@xxxxxxxxxxxx>
> 
> ---
> 
> v3:
>  - Note: I did not added Stefan Reviewed-by since I did more
>    modifications than just a simple respin.
>  - Handle CONFIG_MMU=n as well and rename the key to
>    RISCV_HWPROBE_KEY_HIGHEST_VIRT_ADDRESS
>  - Add a user_max_virt_addr() macro to handle !MMU config
>  - Link to v2: https://lore.kernel.org/lkml/20240220110950.871307-1-cleger@xxxxxxxxxxxx/
> 
> v2:
>  - Note: tried sysconf to export it but this is not backed by syscall
>    and thus does not allow exporting such information easily.
>  - Use arch_get_mmap_end() instead of VA_BITS since it reflects the
>    maximum logical address used by the riscv port
>  - Change hwprobe key name from RISCV_HWPROBE_KEY_VA_BITS to
>    RISCV_HWPROBE_KEY_MAX_ADDRESS
>  - Link to v1: https://lore.kernel.org/lkml/20240201140319.360088-1-cleger@xxxxxxxxxxxx/
> ---
>  Documentation/arch/riscv/hwprobe.rst  | 3 +++
>  arch/riscv/include/asm/hwprobe.h      | 2 +-
>  arch/riscv/include/asm/processor.h    | 8 +++++++-
>  arch/riscv/include/uapi/asm/hwprobe.h | 1 +
>  arch/riscv/kernel/sys_hwprobe.c       | 4 ++++
>  5 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
> index b2bcc9eed9aa..875d3122bd66 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_HIGHEST_VIRT_ADDRESS`: An unsigned long which
> +  represent the highest userspace virtual address usable.
> 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/asm/processor.h b/arch/riscv/include/asm/processor.h
> index a8509cc31ab2..341863d4d989 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -14,7 +14,7 @@
>  
>  #include <asm/ptrace.h>
>  
> -#ifdef CONFIG_64BIT
> +#if defined(CONFIG_64BIT) && defined(CONFIG_MMU)
>  #define DEFAULT_MAP_WINDOW	(UL(1) << (MMAP_VA_BITS - 1))
>  #define STACK_TOP_MAX		TASK_SIZE
>  
> @@ -58,6 +58,12 @@
>  
>  #define STACK_TOP		DEFAULT_MAP_WINDOW
>  
> +#ifdef CONFIG_MMU
> +#define user_max_virt_addr() arch_get_mmap_end(ULONG_MAX, 0, 0)
> +#else
> +#define	user_max_virt_addr() 0
> +#endif /* CONFIG_MMU */
> +
>  /*
>   * This decides where the kernel will search for a free chunk of vm
>   * space during mmap's.
> diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
> index 9f2a8e3ff204..3630e3f30354 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_HIGHEST_VIRT_ADDRESS	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..560ea41a716d 100644
> --- a/arch/riscv/kernel/sys_hwprobe.c
> +++ b/arch/riscv/kernel/sys_hwprobe.c
> @@ -8,6 +8,7 @@
>  #include <asm/cacheflush.h>
>  #include <asm/cpufeature.h>
>  #include <asm/hwprobe.h>
> +#include <asm/processor.h>
>  #include <asm/sbi.h>
>  #include <asm/switch_to.h>
>  #include <asm/uaccess.h>
> @@ -202,6 +203,9 @@ 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_HIGHEST_VIRT_ADDRESS:
> +		pair->value = user_max_virt_addr();
> +		break;
>  
>  	/*
>  	 * For forward compatibility, unknown keys don't fail the whole
> -- 
> 2.43.0
> 

Looks like a great solution.

Reviewed-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>