Re: [PATCH v2] riscv: hwprobe: export largest userspace address

[Clément Léger]

On 20/02/2024 17:50, Stefan O'Rear wrote:
> 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@xxxxxxxxxxxx>
> 
> Reviewed-by: Stefan O'Rear <sorear@xxxxxxxxxxxx>
> 
> Ideally, we'd have something arch-independent, but this is a good interface
> and simple enough to support in perpetuity.

Hi Stefan,

Indeed while not architecture specific, I did not found anything
relevant in which this information could easily fit. sysconf is actually
mainly compile time constants returned by the libc. And the few non
POSIX sysconf defines (_SC_xx) are actually using either /proc parsing
or the sysinfo syscall which did not changed since kernel 2.3. But maybe
someone out there knows something better suited to export this information.

Thanks,

Clément

> 
> -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@xxxxxxxxxxxx/
>> ---
>>  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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