Re: [PATCH 1/4] x86/pagewalk: avoid ACCESS_ONCE() in compound literals

[Roger Pau Monné <roger@xxxxxxxxxxxxxx>]

On Mon, Sep 21, 2026 at 10:59:09AM +0200, Jan Beulich wrote:
> On 21.09.2026 10:53, Roger Pau Monné wrote:
> > On Thu, Sep 03, 2026 at 01:51:28PM +0200, Jan Beulich wrote:
> >> Compound literals are also covered by Misra C:2012 rule 13.1
> >> ("Initializer lists shall not contain persistent side effects"), and
> >> (sadly?) that rule also applies to lists with just a single element, or
> >> more generally with just a single side effect. Use intermediate variables
> >> to overcome this as well as ACCESS_ONCE()'s restriction to be usable on
> >> scalar types only.
> >>
> >> No functional change intended.
> >>
> >> Fixes: 0345b835dc97 ("x86/pagewalk: Read guest PTEs with ACCESS_ONCE()")
> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> >>
> >> --- a/xen/arch/x86/mm/guest_walk.c
> >> +++ b/xen/arch/x86/mm/guest_walk.c
> >> @@ -129,7 +129,9 @@ guest_walk_tables(const struct vcpu *v,
> >>              guest_l4_table_offset(va) * sizeof(gw->l4e);
> >>      if ( !hvmemul_read_cache(v, l4gpa, &gw->l4e, sizeof(gw->l4e)) )
> >>      {
> >> -        gw->l4e = (guest_l4e_t){ 
> >> ACCESS_ONCE(l4p[guest_l4_table_offset(va)].l4) };
> >> +        guest_intpte_t l4e = 
> >> ACCESS_ONCE(l4p[guest_l4_table_offset(va)].l4);
> >> +
> >> +        gw->l4e = (guest_l4e_t){ l4e };
> > 
> > Did you consider using the l4e_from_intpte() and similar helpers here
> > and below?
> 
> No, I didn't, as they aren't applicable to guest_l<N>e_t. guest_intpte_t
> aliases intpte_t only for GUEST_PAGING_LEVELS > 2.

Right, as otherwise it's a 32bit PTE.

Acked-by: Roger Pau Monné <roger@xxxxxxxxxxxxxx>

Thanks, Roger.