Re: [PATCH v6 3/4] xen/riscv: fix A/D bits in Xen's page-table mappings

[Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>]

On 2026-10-14 11:59:43+02:00, Oleksii Kurochko wrote:
> On 10/13/26 12:07 PM, 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).
> > 
> > Without Svadu, 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")
> 
> I think I missed to mentioned that before, sorry:
> 
> arch_pmap_map() was introduced by f89e1a170299 ("xen/riscv: introduce
> asm/pmap.h header"), so perhaps add a 2nd Fixes: tag.
Thanks, will apply this in next version.

-- 
Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>