Re: [Xen-devel] [PATCH RFC] x86/ioreq server: Optimize p2m cleaning up code in p2m_finish_type_change().

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

On 5/10/2017 12:29 AM, Jan Beulich wrote:
> > > > On 05.04.17 at 10:59, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> > --- a/xen/arch/x86/hvm/dm.c
> > +++ b/xen/arch/x86/hvm/dm.c
> > @@ -411,14 +411,17 @@ static int dm_op(domid_t domid,
> >               while ( read_atomic(&p2m->ioreq.entry_count) &&
> >                       first_gfn <= p2m->max_mapped_pfn )
> >               {
> > +                bool changed = false;
> > +
> >                   /* Iterate p2m table for 256 gfns each time. */
> >                   p2m_finish_type_change(d, _gfn(first_gfn), 256,
> > -                                       p2m_ioreq_server, p2m_ram_rw);
> > +                                       p2m_ioreq_server, p2m_ram_rw, &changed);
> >   
> >                   first_gfn += 256;
> >   
> >                   /* Check for continuation if it's not the last iteration. */
> >                   if ( first_gfn <= p2m->max_mapped_pfn &&
> > +                     changed &&
> >                        hypercall_preempt_check() )
> >                   {
> >                       rc = -ERESTART;
> I appreciate and support the intention, but you're opening up a
> long lasting loop here in case very little or no changes need to
> be done. You need to check for preemption every so many
> iterations even if you've never seen "changed" come back set.
Thanks for your comments, Jan.
Indeed, this patch is problematic. Another thought is - since current 
p2m sweeping
implementation disables live migration when there's ioreq server entries 
left, and George
had proposed a generic p2m change solution previously. I'd like to leave 
the optimization
together with the generic solution in future xen release. :-)


Yu

> Jan
>
>
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