Re: [PATCH v8 4/5] xen/riscv: probe the A/D update scheme if the DT advertises neither

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.10.2026 16:35, Oleksii Kurochko wrote:
> On 10/26/26 4:14 PM, Baptiste Le Duc wrote:
>> +/*
>> + * Probe whether hardware updates the PTE A/D bits by writing to a local
>> + * variable through an alias mapping whose A and D bits are clear.
>> + *
>> + * Check the D bit rather than the A bit, as the A bit may be set
>> + * speculatively while the D bit can't.
>> + *
>> + * Under Svade, the store raises a page fault instead, so use the extable to
>> + * handle it.
>> + */
>> +static bool __init probe_hw_ad_update(void)
>> +{
>> +    unsigned int probe_target;
>> +    vaddr_t va = FIXMAP_ADDR(FIX_MISC) + PAGE_OFFSET(&probe_target);
>> +    pte_t *entry = &xen_fixmap[FIX_MISC];
>> +    bool d_set = false;
>> +
>> +    ASSERT(!pte_is_valid(*entry));
>> +
>> +    /*
>> +     * Any page in use will do, so alias the stack page this code runs on,
>> +     * with A and D clear. Store to a variable of its own, so that nothing
>> +     * the compiler relies on gets overwritten.
>> +     */
>> +    write_pte(entry, 
>> pte_from_mfn(maddr_to_mfn(virt_to_maddr(&probe_target)),
>> +                                  PTE_VALID | PTE_READABLE | PTE_WRITABLE));
>> +    flush_tlb_one_local(va);
>> +
>> +    asm_inline goto (
> 
> I think that generally this code should be wrapped by "#ifdef 
> CONFIG_CC_HAS_ASM_GOTO_OUTPUT"

I don't follow why you say this when ...

> but we can keep it as is here as we don't 
> use output operands.

... right afterwards you explain why it isn't needed.

Jan