Re: [Xen-devel] [PATCH 02/13] xen: anchor linear p2m list in shared info structure

[Juergen Gross]

On 02/18/2015 11:54 AM, David Vrabel wrote:
> On 18/02/15 10:50, Andrew Cooper wrote:
> > On 18/02/15 10:42, Juergen Gross wrote:
> > > > >    /* Set up p2m_top to point to the domain-builder provided p2m
> > > > > pages */
> > > > > @@ -469,8 +473,10 @@ static pte_t *alloc_p2m_pmd(unsigned long addr,
> > > > > pte_t *pte_pg)
> > > > >
> > > > >            ptechk = lookup_address(vaddr, &level);
> > > > >            if (ptechk == pte_pg) {
> > > > > +            HYPERVISOR_shared_info->arch.p2m_generation++;
> > > > >                set_pmd(pmdp,
> > > > >                    __pmd(__pa(pte_newpg[i]) | _KERNPG_TABLE));
> > > > > +            HYPERVISOR_shared_info->arch.p2m_generation++;
> > > >
> > > > Do these increments of p2m_generation need to be atomic?
> > >
> > > Hmm, they are done under lock. I don't think the compiler is allowed to
> > > reorder the writes to p2m_generation across set_pmd().
> >
> > They do need smp_wmb() to guarantee that the increment is visible before
> > the update occurs, just as the toolstack will need smp_rmb() to read.
>
> smp_wmb() isn't good enough since you need the barrier even on non-smp
> -- you need a wmb().

Okay, will do.

Juergen

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