[PATCH v8 11/11] riscv: hwprobe: Introduce rva23u64 base behavior

[Guodong Xu]

Hi Radim,

On Tue, 29 Sep 2026 16:00:22 +0000, Radim Krcmar wrote:
> 2026-09-20T03:18:31-04:00, Guodong Xu <guodong.xu at oss.qualcomm.com>:
>> [ ... ]
>> +	int cpu;
>> +
>> +	for_each_cpu(cpu, cpus) {
>> +		if (!riscv_isa_base_available(hart_isa[cpu].isa_bases, base))
>> +			return false;
>
> I think that extensions like F or V might be present in the ISA, yet
> actually disabled in user-mode and this loop would misreport that
> RVA23U64 is present.

Only V has a way to be disabled by user-mode, through
prctl(PR_RISCV_V_SET_CONTROL) with PR_RISCV_V_VSTATE_CTRL_OFF. That
interface is questionable, and obsoleting it was discussed on the
RISE Platform WG call on 16 Sep. And I plan to take this to LPC26
next week in Prague to see what the community says.

Besides, since you mentioned hwprobe_isa_ext0() below: actually, today's
per hart hwprobe_isa_ext0() doesn't honor that prctl(V OFF), IMA_V stays
set. Refer to [1].

There is no way for a user-mode application to disable F/D.

> Is there a reason to diverge from how hwprobe_isa_ext0() does the
> extension detection?  (has_fpu() and other global checks)

Anyway, back to the RVA23U64 isa_base bit. In this design, I mean to
report the per-hart RVA23U64 base, which is derived from hart_isa[cpu].isa.
That bitmap has passed the validate callbacks in riscv_resolve_isa(), so
it "matches kernel configuration as well as correct extension
dependencies".

There can be a mismatch on a system with heterogeneous harts, where
has_fpu() is false globally but this particular hart has F/D. On such
a system Zfa and Zcd are still reported for that hart today, which is
the same mismatch. Same reason, because Zfa/Zcd passed validate callbacks
per hart.

So the isa_base bit RVA23U64 behaves like the per-hart keys do today,
while IMA_FD, IMA_C and IMA_V at the top of hwprobe_isa_ext0() are
host-wide. My take is that when user space queries over all CPUs, it
gets the intersection, where RVA23U64 is not reported in such a
heterogeneous situation. Per hart, I want to keep the kernel config + ISA
string truth.

[1] https://lore.kernel.org/linux-riscv/20260725001614.2578617-1-tchiu@tenstorrent.com/

Guodong