Re: [Xen-devel] [PATCH V2 5/5] Xen: switch to linear virtual mapped sparse p2m list

[Juergen Gross]

On 11/07/2014 03:40 PM, David Vrabel wrote:
> On 07/11/14 14:11, Juergen Gross wrote:
> > On 11/07/2014 02:54 PM, David Vrabel wrote:
> > > On 06/11/14 05:47, Juergen Gross wrote:
> > > [...]
> > > > @@ -526,23 +411,83 @@ unsigned long get_phys_to_machine(unsigned long
> > > > pfn)
> > > >            return IDENTITY_FRAME(pfn);
> > > >        }
> > > >
> > > > -    topidx = p2m_top_index(pfn);
> > > > -    mididx = p2m_mid_index(pfn);
> > > > -    idx = p2m_index(pfn);
> > > > +    ptep = lookup_address((unsigned long)(xen_p2m_addr + pfn), &level);
> > > > +    BUG_ON(!ptep || level != PG_LEVEL_4K);
> > > >
> > > >        /*
> > > >         * The INVALID_P2M_ENTRY is filled in both p2m_*identity
> > > >         * and in p2m_*missing, so returning the INVALID_P2M_ENTRY
> > > >         * would be wrong.
> > > >         */
> > > > -    if (p2m_top[topidx][mididx] == p2m_identity)
> > > > +    if (pte_pfn(*ptep) == PFN_DOWN(__pa(p2m_identity)))
> > > >            return IDENTITY_FRAME(pfn);
> > > >
> > > > -    return p2m_top[topidx][mididx][idx];
> > > > +    return xen_p2m_addr[pfn];
> > >
> > > You should test xen_p2m_addr[pfn] == INVALID_P2M_ENTRY before checking
> > > if it's an identity entry.  This should skip the more expensive
> > > lookup_address() in the common case.
> >
> > I do. The check is in __pfn_to_mfn(). get_phys_to_machine() is called in
> > this case only.
>
> So you do.  I missed that.
>
> > > >    bool __set_phys_to_machine(unsigned long pfn, unsigned long mfn)
> > >
> > > I think you should map p2m_missing and p2m_identity as read-only and do
> > > the new page allocation on a write fault.
> > >
> > > set_phys_to_machine() is used every grant map and unmap and in the
> > > common case (already allocated array page) it must be a fast and simple:
> > >
> > >       xen_p2m_addr[pfn] = mfn;
> >
> > Nice idea. I'll try it.
>
> You probably want to try this with a custom exception fixup (or abuse
> put_user()).

Yeah, that was my idea, too.

Juergen

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