Unverified Commit c9b8cd13 authored by Clément Léger's avatar Clément Léger Committed by Palmer Dabbelt
Browse files

riscv: hwprobe: export highest virtual userspace address 

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: default avatarClément Léger <cleger@rivosinc.com>
Reviewed-by: default avatarCharlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20240410144558.1104006-1-cleger@rivosinc.com


Signed-off-by: default avatarPalmer Dabbelt <palmer@rivosinc.com>
parent 16badacd

Documentation/arch/riscv/hwprobe.rst

+3 −0
Original line number Diff line number Diff line
@@ -257,3 +257,6 @@ 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.

arch/riscv/include/asm/hwprobe.h

+1 −1
Original line number Diff line number Diff line
@@ -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)

{

arch/riscv/include/asm/processor.h

+6 −0
Original line number Diff line number Diff line
@@ -57,6 +57,12 @@ 


#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.

arch/riscv/include/uapi/asm/hwprobe.h

+1 −0
Original line number Diff line number Diff line
@@ -79,6 +79,7 @@ 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 */

arch/riscv/kernel/sys_hwprobe.c

+4 −0
Original line number Diff line number Diff line
@@ -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>
@@ -231,6 +232,9 @@ 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