[PATCH] riscv: hwprobe: export VA_BITS

[Jessica Clarke]

On 3 Feb 2024, at 10:40, Stefan O'Rear <sorear at fastmail.com> wrote:
> 
> On Fri, Feb 2, 2024, at 3:22 AM, Clément Léger wrote:
>> On 02/02/2024 03:32, Charlie Jenkins wrote:
>>> On Thu, Feb 01, 2024 at 03:02:45PM +0100, Clément Léger wrote:
>>>> Some userspace applications (OpenJDK for instance) uses the free bits
>>>> in pointers to insert additional information for their own logic.
>>>> Currently they rely on parsing /proc/cpuinfo to obtain the current value
>>>> of virtual address used bits [1]. Exporting VA_BITS through hwprobe will
>>>> allow for a more stable interface to be used.
>>> 
>>> mmap already supports this without a need for applications to know the
>>> underlying hardware. If a hint address is passed into mmap, it will never
>>> return an address that uses more bits than the hint address. I designed
>>> it that way so that something like this wasn't necessary.
>> 
>> Ok even though probing this kind of thing is probably not what mmap is
>> meant to do. IMHO, probing this through the regular hwprobe interface is
>> probably more coherent but maybe Robin (which needs this information)
>> can rely on that.
> 
> Both of these are useful, separately and in conjunction.
> 
> hwprobe allows applications which can adapt to different VA sizes to learn
> which is in use prior to allocating memory.
> 
> mmap hints allow applications which require a fixed limit on the VA size to
> express that limit at the point of requirement, the hint can be set based on
> the hwprobe result to explicitly indicate its use.

Neither is an architecture-specific concept though. If you want to
expose a notion of VA bits to userspace then it should probably be done
in a more generic manner than the RISC-V-specific hwprobe syscall,
probably as two numbers, low and high bits (ia64’s regions may be gone,
but sparc64 still has both halves of the address space presented to
userspace, with the stack in high/negative memory).

Jess

> -s
> 
>> Clément
>> 
>>> 
>>> - Charlie
>>> 
>>>> 
>>>> 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>
>>>> 
>>>> ---
>>>> 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       | 3 +++
>>>> 4 files changed, 8 insertions(+), 1 deletion(-)
>>>> 
>>>> diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
>>>> index b2bcc9eed9aa..6f198c6ed4f0 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_VA_BITS`: An unsigned long which
>>>> +  represent the number of bits used to store virtual addresses.
>>>> 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..2a5006cddb7b 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_VA_BITS 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..328435836e36 100644
>>>> --- a/arch/riscv/kernel/sys_hwprobe.c
>>>> +++ b/arch/riscv/kernel/sys_hwprobe.c
>>>> @@ -202,6 +202,9 @@ 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_VA_BITS:
>>>> + pair->value = VA_BITS;
>>>> + break;
>>>> 
>>>> /*
>>>>  * For forward compatibility, unknown keys don't fail the whole
>>>> -- 
>>>> 2.43.0
>>>> 
>>>> 
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