Re: [PATCH v10 1/1] xen/riscv: probe the A/D update scheme if the DT advertises neither

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.10.2026 15:21, Oleksii Kurochko wrote:
> 
> 
> On 10/28/26 2:05 PM, Baptiste Le Duc wrote:
>> p2m_set_pte_flags() presets the A/D bits of G-stage PTEs only when
>> RISCV_ISA_EXT_svade is set, which is correct only if the device tree is.
>>
>> The privileged spec allows exactly two schemes for updating the PTE A and D
>> bits: hardware sets them on access and on write, or accessing a page whose
>> A and/or D bit is clear raises a page fault so that software can set them
>> (ratified as Svade). Every implementation uses one of them, but platforms
>> designed before Svade and Svadu were ratified, such as the HiFive Premier
>> P550, may advertise neither.
>>
>> Trust the device tree when it advertises Svade and/or Svadu, as for any
>> other extension. When it advertises neither, probe the scheme in effect
>> and record the matching extension in riscv_isa, so that such device trees
>> can be used as-is, and warn naming the extension to add to "riscv,isa".
>>
>> Introduce probe_hw_ad_update(): alias the first page of the Xen image
>> read-only through the FIX_MISC fixmap slot with its A and D bits clear,
>> then load from it under an exception table fixup. Hardware updating is in
>> effect if the load completes and the A bit is then set. The probe fails
>> otherwise, whether the load faults or completes leaving the A bit clear.
>> The A bit being set by a speculative access still implies Svadu, as Svade
>> only allows A to be set by software.
>>
>> Add Svadu to the known ISA extensions so it can be told apart from Svade.
>> The comment above the enum asks for new extensions to go at the bottom to
>> avoid shuffling, a leftover from Linux where the IDs were macros with
>> explicit values. Nothing depends on the enum values, so have the comment
>> say that multi-letter extensions follow the order of riscv_isa_ext[].
>>
>> Assisted-by: Claude:claude-opus-5-5
>> Signed-off-by: Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>
>> ---
>> Changes since v9:
> 
> Nit: something wrong with version numbering. (in the case of v11 would 
> be nice to fix).
> 
> Changes itself LGTM:
> 
> Reviewed-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>