Re: [RFT PATCH] riscv: errata: mips: Skip vendor extension lookup during early alternatives

[Aleksa Paunovic <aleksa.paunovic@htecgroup.com>]

Hi Karl,

Thanks for the fix!

On 10/16/26 22:30, Karl Mehltretter wrote:
> When an enabled erratum selects RISCV_ALTERNATIVE_EARLY,
> apply_early_boot_alternatives() calls the MIPS errata callback from
> setup_vm() with the MMU disabled. Each enabled P8700 probe calls
> riscv_isa_vendor_extension_available() before rejecting
> RISCV_ALTERNATIVES_EARLY_BOOT, although neither workaround is applied in
> that stage.
>
> KASAN instrumentation in __riscv_isa_vendor_extension_available() reads
> shadow memory with satp=0. On QEMU's mips-p8700 model this raises a load
> access fault, and the temporary trap vector parks the boot hart. The guest
> prints nothing after the OpenSBI handoff.
>
> Move the existing early-stage rejection before the extension lookup in both
> probes. All later stages retain their existing lookup and patch selection.
>
> Fixes: 0b0ca959d206 ("riscv: errata: Fix the PAUSE Opcode for MIPS P8700")
> Fixes: 2cf0b2992203 ("riscv: Add ERRATA_MIPS_P8700_WFI to replace WFI with mips.pause")
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
> ---
> Based on RISC-V for-next commit a5ae1bf65cee.
>
> Tested with QEMU 11.0.2, virt/mips-p8700, four harts, and generic KASAN:
>
> - The parent reached the OpenSBI handoff but produced no Linux output in
>   120 seconds.
> - The patched kernel initialized KASAN, brought all four harts online, and
>   powered off cleanly.
> - With a DTB advertising xmipsexectl, all 414 PAUSE and 6 WFI sites in live
>   kernel text contained mips.pause.
>
> QEMU does not model the physical WFI translation erratum or P8700 PAUSE
> behavior. This was not tested on P8700 hardware. Please test with KASAN and
> both P8700 errata enabled.
I tested this on a Boston board configured with a single-core MIPS P8700 CPU. 
The kernel was built with KASAN_INLINE enabled. Reproduced the bug and confirmed
that your fix works. FWIW, I was not able to reproduce it with KASAN_OUTLINE.

Tested-by: Aleksa Paunovic <aleksa.paunovic@htecgroup.com>

Best regards,
Aleksa