Re: [PATCH v3 RESEND] riscv: hwprobe: allow querying available RISCV_HWPROBE_KEY_IMA_EXT_0 bits

[Charlie Jenkins <thecharlesjenkins@gmail.com>]

On Mon, Aug 25, 2025 at 05:00:45PM -0500, Andrew Jones wrote:
> On Mon, Aug 25, 2025 at 08:02:21PM +0300, offtkp wrote:
> > When probing for extensions using RISCV_HWPROBE_KEY_IMA_EXT_0, a missing
> > bit in the resulting bitmask means the extension is not available or the
> > kernel is not recent enough to support the bit. Currently, there's no
> > way to differentiate between the two.
> > 
> > This adds a new riscv_hwprobe key, RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL,
> > which returns a bitmask of all the queryable extensions supported by the
> > kernel in RISCV_HWPROBE_KEY_IMA_EXT_0. This can allow programs to use a
> > fallback extension detection method when the bit they want to query is
> > not available in the kernel they are running on.
> > 
> > Signed-off-by: offtkp <parisoplop@gmail.com>
> > ---
> > Changes in v3:
> > - remove the 59 magic number and create the mask as a define
> > 
> > Conversation from v2 to show motivation for this patch as a whole:
> > 
> > > And do you have a real usecase in
> > > mind? I mean which project would benefit from this?
> > Yes, I want this feature for an x86-on-RISC-V userspace emulator
> > called felix86. We do runtime extension detection for our JIT.
> > Currently, for any extension that defines instructions we use the SIGILL
> > method previously described, because there's no way to tell with just
> > a hwprobe call whether a 0 bit means extension not present or kernel
> > too old. But we can't do this for all extensions, some don't define
> > instructions.
> > 
> > The end goal would be being able to detect an extension using hwprobe,
> > and if it's not detectable, notifying the user that we can't detect it and
> > they can mark it as enabled in the emulator's config file.
> 
> As I pointed out on v2, I don't think this patch helps. When the kernel
> is unaware of the extension, then we still know nothing about its
> availability. This is only helpful when the kernel is aware of an
> extension, but, in those cases, the current hwprobe can be trusted, so
> we don't need this.

You are correct to say that when the kernel reports that an extension as
"0" that there is no way for the userspace application to be able to
use the extension, the kernel reporting as "unaware" does not help with
this.

This patch from Paris, however, is not about dynamically increasing the
number of extensions that a userspace application can use but rather
simply about error/debug reporting.

I think this kind of reporting is a valid usecase for a hwprobe feature
and would be able to improve user experience of userspace applications.

- Charlie

> 
> The problem is there's no good way to detect extensions that the kernel
> is unaware of, which means the kernel needs to blindly expose what's in
> the DT/ACPI somehow.
> 
> Thanks,
> drew
> 
> > 
> > 
> > 
> >  Documentation/arch/riscv/hwprobe.rst  |  5 ++++-
> >  arch/riscv/include/asm/hwprobe.h      | 29 ++++++++++++++++++++++++++-
> >  arch/riscv/include/uapi/asm/hwprobe.h |  2 ++
> >  arch/riscv/kernel/sys_hwprobe.c       |  4 ++++
> >  4 files changed, 38 insertions(+), 2 deletions(-)
> > 
> > diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
> > index 2aa9be272d5d..6d77def0a46e 100644
> > --- a/Documentation/arch/riscv/hwprobe.rst
> > +++ b/Documentation/arch/riscv/hwprobe.rst
> > @@ -360,4 +360,7 @@ The following keys are defined:
> >  
> >      * :c:macro:`RISCV_HWPROBE_VENDOR_EXT_XSFVFWMACCQQQ`: The Xsfvfwmaccqqq
> >          vendor extension is supported in version 1.0 of Matrix Multiply Accumulate
> > -	Instruction Extensions Specification.
> > \ No newline at end of file
> > +	Instruction Extensions Specification.
> > +
> > +* :c:macro:`RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL`: A bitmask containing the extensions
> > +  that can be probed using the :c:macro:`RISCV_HWPROBE_KEY_IMA_EXT_0` key.
> > \ No newline at end of file
> > diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
> > index 7fe0a379474a..a06eeec11e2c 100644
> > --- a/arch/riscv/include/asm/hwprobe.h
> > +++ b/arch/riscv/include/asm/hwprobe.h
> > @@ -8,7 +8,34 @@
> >  
> >  #include <uapi/asm/hwprobe.h>
> >  
> > -#define RISCV_HWPROBE_MAX_KEY 13
> > +#define RISCV_HWPROBE_MAX_KEY 14
> > +#define RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL_VALUE	(RISCV_HWPROBE_IMA_FD | \
> > +	RISCV_HWPROBE_IMA_C | RISCV_HWPROBE_IMA_V | RISCV_HWPROBE_EXT_ZBA | \
> > +	RISCV_HWPROBE_EXT_ZBB | RISCV_HWPROBE_EXT_ZBS | RISCV_HWPROBE_EXT_ZICBOZ | \
> > +	RISCV_HWPROBE_EXT_ZBC | RISCV_HWPROBE_EXT_ZBKB | RISCV_HWPROBE_EXT_ZBKC | \
> > +	RISCV_HWPROBE_EXT_ZBKX | RISCV_HWPROBE_EXT_ZKND | RISCV_HWPROBE_EXT_ZKNE | \
> > +	RISCV_HWPROBE_EXT_ZKNH | RISCV_HWPROBE_EXT_ZKSED | \
> > +	RISCV_HWPROBE_EXT_ZKSH | RISCV_HWPROBE_EXT_ZKT | RISCV_HWPROBE_EXT_ZVBB | \
> > +	RISCV_HWPROBE_EXT_ZVBC | RISCV_HWPROBE_EXT_ZVKB | RISCV_HWPROBE_EXT_ZVKG | \
> > +	RISCV_HWPROBE_EXT_ZVKNED | RISCV_HWPROBE_EXT_ZVKNHA | \
> > +	RISCV_HWPROBE_EXT_ZVKNHB | RISCV_HWPROBE_EXT_ZVKSED | \
> > +	RISCV_HWPROBE_EXT_ZVKSH | RISCV_HWPROBE_EXT_ZVKT | \
> > +	RISCV_HWPROBE_EXT_ZFH | RISCV_HWPROBE_EXT_ZFHMIN | \
> > +	RISCV_HWPROBE_EXT_ZIHINTNTL | RISCV_HWPROBE_EXT_ZVFH | \
> > +	RISCV_HWPROBE_EXT_ZVFHMIN | RISCV_HWPROBE_EXT_ZFA | \
> > +	RISCV_HWPROBE_EXT_ZTSO | RISCV_HWPROBE_EXT_ZACAS | \
> > +	RISCV_HWPROBE_EXT_ZICOND | RISCV_HWPROBE_EXT_ZIHINTPAUSE | \
> > +	RISCV_HWPROBE_EXT_ZVE32X | RISCV_HWPROBE_EXT_ZVE32F | \
> > +	RISCV_HWPROBE_EXT_ZVE64X | RISCV_HWPROBE_EXT_ZVE64F | \
> > +	RISCV_HWPROBE_EXT_ZVE64D | RISCV_HWPROBE_EXT_ZIMOP | \
> > +	RISCV_HWPROBE_EXT_ZCA | RISCV_HWPROBE_EXT_ZCB | RISCV_HWPROBE_EXT_ZCD | \
> > +	RISCV_HWPROBE_EXT_ZCF | RISCV_HWPROBE_EXT_ZCMOP | \
> > +	RISCV_HWPROBE_EXT_ZAWRS | RISCV_HWPROBE_EXT_SUPM | \
> > +	RISCV_HWPROBE_EXT_ZICNTR | RISCV_HWPROBE_EXT_ZIHPM | \
> > +	RISCV_HWPROBE_EXT_ZFBFMIN | RISCV_HWPROBE_EXT_ZVFBFMIN | \
> > +	RISCV_HWPROBE_EXT_ZVFBFWMA | RISCV_HWPROBE_EXT_ZICBOM | \
> > +	RISCV_HWPROBE_EXT_ZAAMO | RISCV_HWPROBE_EXT_ZALRSC | \
> > +	RISCV_HWPROBE_EXT_ZABHA)
> >  
> >  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 aaf6ad970499..a3b92df4dc05 100644
> > --- a/arch/riscv/include/uapi/asm/hwprobe.h
> > +++ b/arch/riscv/include/uapi/asm/hwprobe.h
> > @@ -82,6 +82,7 @@ struct riscv_hwprobe {
> >  #define		RISCV_HWPROBE_EXT_ZAAMO		(1ULL << 56)
> >  #define		RISCV_HWPROBE_EXT_ZALRSC	(1ULL << 57)
> >  #define		RISCV_HWPROBE_EXT_ZABHA		(1ULL << 58)
> > +/* Change RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL_VALUE when adding items. */
> >  #define RISCV_HWPROBE_KEY_CPUPERF_0	5
> >  #define		RISCV_HWPROBE_MISALIGNED_UNKNOWN	(0 << 0)
> >  #define		RISCV_HWPROBE_MISALIGNED_EMULATED	(1 << 0)
> > @@ -106,6 +107,7 @@ struct riscv_hwprobe {
> >  #define RISCV_HWPROBE_KEY_VENDOR_EXT_THEAD_0	11
> >  #define RISCV_HWPROBE_KEY_ZICBOM_BLOCK_SIZE	12
> >  #define RISCV_HWPROBE_KEY_VENDOR_EXT_SIFIVE_0	13
> > +#define RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL 14
> >  /* 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 0b170e18a2be..40e7fa5f85f3 100644
> > --- a/arch/riscv/kernel/sys_hwprobe.c
> > +++ b/arch/riscv/kernel/sys_hwprobe.c
> > @@ -310,6 +310,10 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> >  		hwprobe_isa_vendor_ext_thead_0(pair, cpus);
> >  		break;
> >  
> > +	case RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL:
> > +		pair->value = RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL_VALUE;
> > +		break;
> > +
> >  	/*
> >  	 * For forward compatibility, unknown keys don't fail the whole
> >  	 * call, but get their element key set to -1 and value set to 0
> > 
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> 
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