Re: [Xen-devel] [PATCH v9 4/5] x86/ioreq server: Asynchronously reset outstanding p2m_ioreq_server entries.

[Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>]

On 3/22/2017 10:29 PM, Jan Beulich wrote:
> > > > On 21.03.17 at 03:52, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> > --- a/xen/arch/x86/hvm/ioreq.c
> > +++ b/xen/arch/x86/hvm/ioreq.c
> > @@ -949,6 +949,14 @@ int hvm_map_mem_type_to_ioreq_server(struct domain *d, 
> > ioservid_t id,
> >   
> >       spin_unlock_recursive(&d->arch.hvm_domain.ioreq_server.lock);
> >   
> > +    if ( rc == 0 && flags == 0 )
> > +    {
> > +        struct p2m_domain *p2m = p2m_get_hostp2m(d);
> > +
> > +        if ( read_atomic(&p2m->ioreq.entry_count) )
> > +            p2m_change_entry_type_global(d, p2m_ioreq_server, p2m_ram_rw);
> > +    }
> If you do this after dropping the lock, don't you risk a race with
> another server mapping the type to itself?
I believe it's OK. Remaining p2m_ioreq_server entries still needs to be 
cleaned anyway.
> > --- a/xen/arch/x86/mm/p2m-ept.c
> > +++ b/xen/arch/x86/mm/p2m-ept.c
> > @@ -544,6 +544,12 @@ static int resolve_misconfig(struct p2m_domain *p2m,
> > unsigned long gfn)
> >                       e.ipat = ipat;
> >                       if ( e.recalc && p2m_is_changeable(e.sa_p2mt) )
> >                       {
> > +                         if ( e.sa_p2mt == p2m_ioreq_server )
> > +                         {
> > +                             p2m->ioreq.entry_count--;
> > +                             ASSERT(p2m->ioreq.entry_count >= 0);
> If you did the ASSERT() first (using > 0), you wouldn't need the
> type be a signed one, doubling the valid value range (even if
> right now the full 64 bits can't be used anyway, but it would be
> one less thing to worry about once we get 6-level page tables).
Well, entry_count counts only for 4K pages, so even if the guest 
physical address
width is extended up to 64 bit in the future, entry_count will not 
exceed 2^52(
2^64/2^12).

Yu
> Jan

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