Re: [PATCH v5 2/7] xen/riscv: set up fixmap mappings

[oleksii.kurochko@xxxxxxxxx]

On Tue, 2024-08-27 at 12:29 +0200, Jan Beulich wrote:
> > 
> > +
> > +/*
> > + * Direct access to xen_fixmap[] should only happen when {set,
> > + * clear}_fixmap() is unusable (e.g. where we would end up to
> > + * recursively call the helpers).
> > + */
> > +extern pte_t xen_fixmap[];
> 
> I'm afraid I keep being irritated by the comment: What recursive use
> of
> helpers is being talked about here? I can't see anything recursive in
> this
> patch. If this starts happening with a subsequent patch, then you
> have
> two options: Move the declaration + comment there, or clarify in the
> description (in enough detail) what this is about.
This comment is added because of:
```
void *__init pmap_map(mfn_t mfn)
  ...
       /*
        * We cannot use set_fixmap() here. We use PMAP when the domain map
        * page infrastructure is not yet initialized, so
   map_pages_to_xen() called
        * by set_fixmap() needs to map pages on demand, which then calls
   pmap()
        * again, resulting in a loop. Modify the PTEs directly instead.
   The same
        * is true for pmap_unmap().
        */
       arch_pmap_map(slot, mfn);
   ...
```
And it happens because set_fixmap() could be defined using generic PT
helpers so what will lead to recursive behaviour when when there is no
direct map:
   static pte_t *map_table(mfn_t mfn)
   {
       /*
        * During early boot, map_domain_page() may be unusable. Use the
        * PMAP to map temporarily a page-table.
        */
       if ( system_state == SYS_STATE_early_boot )
           return pmap_map(mfn);
       ...
   }

> 
> > @@ -81,6 +82,18 @@ static inline void flush_page_to_ram(unsigned
> > long mfn, bool sync_icache)
> >      BUG_ON("unimplemented");
> >  }
> >  
> > +/* Write a pagetable entry. */
> > +static inline void write_pte(pte_t *p, pte_t pte)
> > +{
> > +    write_atomic(p, pte);
> > +}
> > +
> > +/* Read a pagetable entry. */
> > +static inline pte_t read_pte(pte_t *p)
> > +{
> > +    return read_atomic(p);
> 
> This only works because of the strange type trickery you're playing
> in
> read_atomic(). Look at x86 code - there's a strict expectation that
> the
> type can be converted to/from unsigned long. And page table accessors
> are written with that taken into consideration. Same goes for
> write_pte()
> of course, with the respective comment on the earlier patch in mind.
I will check x86 code. Probably my answer on the patch with
read/write_atomic() suggest that too. Based on the answers to that
patch I think we can go with x86 approach.

Thanks.

~ Oleksii

> 
> Otoh I see that Arm does something very similar. If you have a strong
> need / desire to follow that, then please at least split the two
> entirely separate aspects that patch 1 presently changes both in one
> go.
> 
> Jan