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Message-ID: <ZfD9JV51GER/8OOX@ghost>
Date: Tue, 12 Mar 2024 18:11:01 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Clément Léger <cleger@...osinc.com>
Cc: Jonathan Corbet <corbet@....net>,
Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>, linux-doc@...r.kernel.org,
linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
Robin Ehn <rehn@...osinc.com>, Stefan O'Rear <sorear@...tmail.com>,
Jessica Clarke <jrtc27@...c27.com>
Subject: Re: [PATCH] riscv: hwprobe: export highest virtual userspace address
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@...osinc.com>
>
> ---
>
> 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@rivosinc.com/
>
> 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/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@...osinc.com>
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