Re: [PATCH v7 2/4] xen/riscv: fix A/D bits in Xen's page-table mappings

[Jan Beulich <jbeulich@suse.com>]

On 16.10.2026 17:34, Baptiste Le Duc wrote:
> Xen does not handle page faults caused by clear A/D bits, so it presets
> them when creating PTEs. pt_update_entry() does so, but three places build
> leaf PTEs directly without going through it: setup_initial_mapping() (the
> boot page tables), check_pgtbl_mode_support() (the temporary root entry
> used to probe SATP mode support) and arch_pmap_map() (the fixmap).
> 
> On implementations that fault on clear A/D bits (Svade), all three fault
> on first access. For check_pgtbl_mode_support(), the fault is raised by the
> instruction fetch right after the SATP write, before any trap handler is
> set.
> 
> Add PTE_ACCESSED and PTE_DIRTY to PAGE_HYPERVISOR_RO, and build
> PAGE_HYPERVISOR_RW and PAGE_HYPERVISOR_RX on top of it. PTE_DIRTY is set in
> all cases for consistency with pt_update_entry() which sets it at runtime.
> 
> This fixes arch_pmap_map() for free, since it already builds its PTE from
> PAGE_HYPERVISOR_RW. Switch setup_initial_mapping() to use these macros for
> its default, text and rodata permissions, and check_pgtbl_mode_support() to
> use PAGE_HYPERVISOR_RX for its temporary root entry. The latter drops
> PTE_WRITABLE, going from RWX to RX, but this is harmless, as that entry
> only has to make the current instruction stream fetchable between the two
> CSR_SATP writes, and nothing writes through it.
> 
> Drop the now-redundant PTE_LEAF_DEFAULT, since converting the last
> open-coded site above leaves it with no user outside page.h itself.
> 
> pte_is_table() and pte_is_mapping() both assert that a PTE doesn't use one
> of the two reserved encodings, (V=1, W=1, R=0) and (V=1, X=1, W=1, R=0), by
> masking it with PAGE_HYPERVISOR_RW. Now that PAGE_HYPERVISOR_RW also
> carries A and D, a reserved PTE with A or D set would no longer be caught.
> Mask with the V, R and W bits explicitly instead, and factor the check out
> into pte_has_reserved_rwx().
> 
> Fixes: e66003e7be19 ("xen/riscv: introduce setup_initial_pages")
> Fixes: f89e1a170299 ("xen/riscv: introduce asm/pmap.h header")
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Baptiste Le Duc <baptiste.le-duc@vates.tech>
> Reviewed-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

Acked-by: Jan Beulich <jbeulich@suse.com>