[Bug 220795] riscv_hwprobe and AT_HWCAP diverge after PR_RISCV_V_VSTATE_CTRL_OFF

[Conor Dooley]

Yo

I got added to some bugzilla thing last week about the vector prctl and
how it interacts with hwprobe. I have no idea why I alone was chosen,
since I didn't write this stuff, so I'm linking the issue here so that
someone that does understand it can reply:
https://bugzilla.kernel.org/show_bug.cgi?id=220795

The content of that is:
| Overview:
| 
| On RISC-V systems with Vector support, invoking
| 
| prctl(PR_RISCV_V_SET_CONTROL, PR_RISCV_V_VSTATE_CTRL_OFF)
| 
| 
| disables execution of V instructions for subsequently executed ELF programs, as expected.
| 
| However, after this call:
| 
| AT_HWCAP no longer reports the V bit, indicating that Vector execution is not allowed for new programs, while
| 
| riscv_hwprobe() continues to report RISCV_HWPROBE_IMA_V as present.
| 
| At the same time, multiple downstream user-space components (such as libgcc [1]) already treat riscv_hwprobe as the authoritative source of ISA capability information, since AT_HWCAP has historically been polluted by inconsistent vendor reporting for RVV and cannot represent multi-letter extensions (V, Zve*, Zvfh*, etc.).
| 
| As a result, downstream runtimes may select RVV-enabled code paths even when V has been disabled for the program by system policy.
| 
| To proceed with downstream libc RVV multiversioning work, we would like to confirm whether this divergence between AT_HWCAP and riscv_hwprobe() after PR_RISCV_V_VSTATE_CTRL_OFF is intended (by design), or if it should be considered an unintended semantic inconsistency.
| 
| Additional Context:
| 
| This concern has been raised before in prior discussions [2], noting that hwprobe reports capability while AT_HWCAP reflects availability, creating ambiguity about which interface user space should rely on.
| 
| Reproduce Log:
| [init] ./rvv_check
| AT_HWCAP = 0x000000000020112d
| V via HWCAP:   yes
| hwprobe KEY_IMA_EXT_0 = 0x038d2bf1200000ff
| V via hwprobe: yes
| [init] ./rvv_off rvv_check
| PR_RISCV_V_GET_CONTROL = 0x16
| AT_HWCAP = 0x000000000000112d
| V via HWCAP:   no
| hwprobe KEY_IMA_EXT_0 = 0x038d2bf1200000ff
| V via hwprobe: yes
| [init] poweroff
| [    1.880906] reboot: Power down
| 
| References:
| [1] https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libgcc/config/riscv/feature_bits.c;h=157bcdcad69a939037f6faca3a90c2598dd61f89;hb=HEAD
| [2] https://lists.infradead.org/pipermail/kvm-riscv/2023-June/003915.html

Cheers,
Conor.
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