Re: [Xen-devel] [PATCH v3 2/6] x86 / p2m: remove page_list check in p2m_alloc_table

["Durrant, Paul" <pdurrant@xxxxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 06 March 2020 11:46
> To: pdurrant@xxxxxxxx
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Durrant, Paul <pdurrant@xxxxxxxxxxxx>; 
> Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>; George Dunlap <george.dunlap@xxxxxxxxxx>; Wei 
> Liu <wl@xxxxxxx>; Roger Pau
> Monné <roger.pau@xxxxxxxxxx>
> Subject: Re: [PATCH v3 2/6] x86 / p2m: remove page_list check in 
> p2m_alloc_table
> 
> On 05.03.2020 13:45, pdurrant@xxxxxxxx wrote:
> > From: Paul Durrant <pdurrant@xxxxxxxxxx>
> >
> > There does not seem to be any justification for refusing to create the
> > domain's p2m table simply because it may have assigned pages.
> 
> I think there is: If any such allocation had happened before, how
> would it be represented in the domain's p2m?

Insertion into the p2m is a separate action from page allocation. Why should 
they be linked?

> 
> > Particularly
> > it prevents the prior allocation of PGC_extra pages.
> 
> That's unfortunate, but will need taking care of differently then:
> 
> > --- a/xen/arch/x86/mm/p2m.c
> > +++ b/xen/arch/x86/mm/p2m.c
> > @@ -695,14 +695,6 @@ int p2m_alloc_table(struct p2m_domain *p2m)
> >
> >      p2m_lock(p2m);
> >
> > -    if ( p2m_is_hostp2m(p2m)
> > -         && !page_list_empty(&d->page_list) )
> > -    {
> > -        P2M_ERROR("dom %d already has memory allocated\n", d->domain_id);
> > -        p2m_unlock(p2m);
> > -        return -EINVAL;
> > -    }
> 
> Instead of checking the list to be empty, how about checking
> domain_tot_pages() to return zero?

I could do that, and in fact my original code did, but with more consideration 
the whole test just didn't make sense to me. Yes, clearly the p2m has to be 
there before pages can be added into it, but I can't see any reason why you 
couldn't even allocate the entire guest RAM, then create the p2m and then add 
the pages into it.

  Paul

> 
> Jan
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