Re: [PATCH v6 1/4] xen/riscv: probe the PTE A/D bits update scheme at boot

[Jan Beulich <jbeulich@suse.com>]

On 15.10.2026 10:41, Jan Beulich wrote:
> On 15.10.2026 10:28, Baptiste Le Duc wrote:
>> On 2026-10-15 09:14 +0200, Jan Beulich wrote:
>>> On 14.10.2026 18:45, Baptiste Le Duc wrote:
>>>> On 2026-10-14 11:09:58+02:00, Jan Beulich wrote:
>>>>> On 14.10.2026 11:00, Baptiste Le Duc wrote:
>>>>>
>>>>> In which case it would need to be
>>>>>
>>>>>     if ( riscv_isa_extension_available(NULL, RISCV_ISA_EXT_svade) ||
>>>>>          !riscv_isa_extension_available(NULL, RISCV_ISA_EXT_svadu) )
>>>>>         e->pte |= PTE_ACCESSED | PTE_DIRTY;
>>>>>
>>>>> albeit I have to admit that permitting two contradicting extensions to
>>>>> be exposed at the same time is irritating.
>>>>>
>>>>>> I see two ways to address this:
>>>>>>
>>>>>> 1) Keep checking Svade:
>>>>>>
>>>>>>     if ( riscv_isa_extension_available(NULL, RISCV_ISA_EXT_svade) )
>>>>>>         e->pte |= PTE_ACCESSED | PTE_DIRTY;
>>>>>>
>>>>>>     After resolve_ad_scheme(), the Svade bit is set in every case where
>>>>>>     software has to preset the A/D bits. The only imprecise case is both
>>>>>>     extensions advertised with hardware updating already enabled by
>>>>>>     firmware, where presetting A/D is unnecessary but harmless.
>>>>>>
>>>>>> 2) Record the probe result in a global, e.g.
>>>>>>
>>>>>>     static bool __ro_after_init hw_ad_update;
>>>>>>
>>>>>>     and use:
>>>>>>
>>>>>>     if ( !hw_ad_update )
>>>>>>         e->pte |= PTE_ACCESSED | PTE_DIRTY;
>>>>>>
>>>>>>     This is exact in all cases, at the cost of a new global that mostly
>>>>>>     duplicates what resolve_ad_scheme() already records in the ISA bitmap.
>>>>>>
>>>>>> I discussed with Oleksii and we are fine with either. What do you prefer?
>>>>>
>>>>> As to 1), see above. For 2), while maybe okay, I think we're better off
>>>> Sorry, I don't fully follow the issue with 1). Compared to what you proposed,
>>>> it only differs when neither Svade nor Svadu is recorded, which cannot
>>>> happen as resolve_ad_scheme() runs before and always records one of the
>>>> two.
>>>
>>> Yet previously we said that recording extensions (as such, i.e. later also
>>> possibly exposing them to guests) when they weren't really advertised may
>>> not be the right thing to do. I.e. imo the probing may be used to drive
>> But we also said that platforms (such as the P550) may implement an
>> extension without advertising it, as a RISC-V spec maintainer
>> confirmed in [1].
>>
>> [1] https://github.com/riscv/riscv-isa-manual/issues/3450#issuecomment-6076851535
> 
> What Andrew says there suggests to me that the behavior you observe on P550
> is against the spec.

Or wait - no, he doesn't. He said this only to the strange combination you had
indicated.

Jan