[PATCH v2] riscv: hwprobe: export largest userspace address
Stefan O'Rear
sorear at fastmail.com
Tue Feb 20 08:50:19 PST 2024
On Tue, Feb 20, 2024, at 6:09 AM, 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 used bits [1]. Since this reflect the raw MMU mode
> supported, it might differ from the logical one used internally.
> Exporting the maximum mmappable address through hwprobe will allow a
> more stable interface to be used. For that purpose, add a new hwprobe
> key named RISCV_HWPROBE_MAX_KEY which will export the maximum mmappable
> userspace 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 at rivosinc.com>
Reviewed-by: Stefan O'Rear <sorear at fastmail.com>
Ideally, we'd have something arch-independent, but this is a good interface
and simple enough to support in perpetuity.
-s
> ---
> 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@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 | 4 ++++
> 4 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/arch/riscv/hwprobe.rst
> b/Documentation/arch/riscv/hwprobe.rst
> index b2bcc9eed9aa..a626aa21ac74 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_MAX_USER_ADDRESS`: An unsigned long which
> + represent the maximum userspace mmappable address.
> 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..a6da434be9da 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_MAX_USER_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..19a47540b4a2 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_MAX_USER_ADDRESS:
> + pair->value = arch_get_mmap_end(ULONG_MAX, 0, 0);
> + break;
>
> /*
> * For forward compatibility, unknown keys don't fail the whole
> --
> 2.43.0
>
>
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