Re: [PATCH v4 3/6] RISC-V: hwprobe: Add support for RISCV_HWPROBE_BASE_BEHAVIOR_IMA

[Heiko Stübner]

Hi Evan,

Am Dienstag, 14. März 2023, 19:32:17 CET schrieb Evan Green:
> We have an implicit set of base behaviors that userspace depends on,
> which are mostly defined in various ISA specifications.
> 
> Co-developed-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> Signed-off-by: Evan Green <evan@xxxxxxxxxxxx>
> Reviewed-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> ---
> 
> Changes in v4:
>  - More newlines in BASE_BEHAVIOR_IMA documentation (Conor)
> 
> Changes in v3:
>  - Refactored base ISA behavior probe to allow kernel probing as well,
>    in prep for vDSO data initialization.
>  - Fixed doc warnings in IMA text list, use :c:macro:.
> 
>  Documentation/riscv/hwprobe.rst       | 24 ++++++++++++++++++++++++
>  arch/riscv/include/asm/hwprobe.h      |  2 +-
>  arch/riscv/include/uapi/asm/hwprobe.h |  5 +++++
>  arch/riscv/kernel/sys_riscv.c         | 20 ++++++++++++++++++++
>  4 files changed, 50 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/riscv/hwprobe.rst b/Documentation/riscv/hwprobe.rst
> index 211828f706e3..945d44683c40 100644
> --- a/Documentation/riscv/hwprobe.rst
> +++ b/Documentation/riscv/hwprobe.rst
> @@ -39,3 +39,27 @@ The following keys are defined:
>  
>  * :c:macro:`RISCV_HWPROBE_KEY_MIMPLID`: Contains the value of ``mimplid``, as
>    defined by the RISC-V privileged architecture specification.
> +
> +* :c:macro:`RISCV_HWPROBE_KEY_BASE_BEHAVIOR`: A bitmask containing the base
> +  user-visible behavior that this kernel supports.  The following base user ABIs
> +  are defined:
> +
> +  * :c:macro:`RISCV_HWPROBE_BASE_BEHAVIOR_IMA`: Support for rv32ima or
> +    rv64ima, as defined by version 2.2 of the user ISA and version 1.10 of the
> +    privileged ISA, with the following known exceptions (more exceptions may be
> +    added, but only if it can be demonstrated that the user ABI is not broken):
> +
> +    * The :fence.i: instruction cannot be directly executed by userspace
> +      programs (it may still be executed in userspace via a
> +      kernel-controlled mechanism such as the vDSO).
> +
> +* :c:macro:`RISCV_HWPROBE_KEY_IMA_EXT_0`: A bitmask containing the extensions
> +  that are compatible with the :c:macro:`RISCV_HWPROBE_BASE_BEHAVIOR_IMA`:
> +  base system behavior.
> +
> +  * :c:macro:`RISCV_HWPROBE_IMA_FD`: The F and D extensions are supported, as
> +    defined by commit cd20cee ("FMIN/FMAX now implement
> +    minimumNumber/maximumNumber, not minNum/maxNum") of the RISC-V ISA manual.
> +
> +  * :c:macro:`RISCV_HWPROBE_IMA_C`: The C extension is supported, as defined
> +    by version 2.2 of the RISC-V ISA manual.

just wondering, is there a plan on how further extensions should be added this this?
[as we have this big plethora of them :-) ]

Aka things like Zbb and friends will probably also be relevant to userspace, so just
fill up RISCV_HWPROBE_KEY_IMA_EXT_0 with more elements and once full switch to
RISCV_HWPROBE_KEY_IMA_EXT_1 , RISCV_HWPROBE_KEY_IMA_EXT_2, etc?

Or do we have some more elaborate sorting mechanism?


Thanks
Heiko



> diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
> index 08d1c3bdd78a..7e52f1e1fe10 100644
> --- a/arch/riscv/include/asm/hwprobe.h
> +++ b/arch/riscv/include/asm/hwprobe.h
> @@ -8,6 +8,6 @@
>  
>  #include <uapi/asm/hwprobe.h>
>  
> -#define RISCV_HWPROBE_MAX_KEY 2
> +#define RISCV_HWPROBE_MAX_KEY 4
>  
>  #endif
> diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
> index 591802047460..fc5665411782 100644
> --- a/arch/riscv/include/uapi/asm/hwprobe.h
> +++ b/arch/riscv/include/uapi/asm/hwprobe.h
> @@ -20,6 +20,11 @@ struct riscv_hwprobe {
>  #define RISCV_HWPROBE_KEY_MVENDORID	0
>  #define RISCV_HWPROBE_KEY_MARCHID	1
>  #define RISCV_HWPROBE_KEY_MIMPID	2
> +#define RISCV_HWPROBE_KEY_BASE_BEHAVIOR	3
> +#define		RISCV_HWPROBE_BASE_BEHAVIOR_IMA	(1 << 0)
> +#define RISCV_HWPROBE_KEY_IMA_EXT_0	4
> +#define		RISCV_HWPROBE_IMA_FD		(1 << 0)
> +#define		RISCV_HWPROBE_IMA_C		(1 << 1)
>  /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
>  
>  #endif
> diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
> index 981d23457f13..1c118438b1b3 100644
> --- a/arch/riscv/kernel/sys_riscv.c
> +++ b/arch/riscv/kernel/sys_riscv.c
> @@ -9,6 +9,7 @@
>  #include <asm/cacheflush.h>
>  #include <asm/hwprobe.h>
>  #include <asm/sbi.h>
> +#include <asm/switch_to.h>
>  #include <asm/uaccess.h>
>  #include <asm/unistd.h>
>  #include <asm-generic/mman-common.h>
> @@ -125,6 +126,25 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
>  	case RISCV_HWPROBE_KEY_MIMPID:
>  		hwprobe_arch_id(pair, cpus);
>  		break;
> +	/*
> +	 * The kernel already assumes that the base single-letter ISA
> +	 * extensions are supported on all harts, and only supports the
> +	 * IMA base, so just cheat a bit here and tell that to
> +	 * userspace.
> +	 */
> +	case RISCV_HWPROBE_KEY_BASE_BEHAVIOR:
> +		pair->value = RISCV_HWPROBE_BASE_BEHAVIOR_IMA;
> +		break;
> +
> +	case RISCV_HWPROBE_KEY_IMA_EXT_0:
> +		pair->value = 0;
> +		if (has_fpu())
> +			pair->value |= RISCV_HWPROBE_IMA_FD;
> +
> +		if (elf_hwcap & RISCV_ISA_EXT_c)
> +			pair->value |= RISCV_HWPROBE_IMA_C;
> +
> +		break;
>  
>  	/*
>  	 * For forward compatibility, unknown keys don't fail the whole
>