Re: [Xen-devel] [PATCH v4] xen/grant-table: Avoid m2p_override during mapping

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 22 Jan 2014, Zoltan Kiss wrote:
> > > > > diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> > > > > index 2ae8699..0060178 100644
> > > > > --- a/arch/x86/xen/p2m.c
> > > > > +++ b/arch/x86/xen/p2m.c
> > > > > @@ -872,15 +872,13 @@ static unsigned long mfn_hash(unsigned long mfn)
> > > > > 
> > > > >    /* Add an MFN override for a particular page */
> > > > >    int m2p_add_override(unsigned long mfn, struct page *page,
> > > > > -             struct gnttab_map_grant_ref *kmap_op)
> > > > > +             struct gnttab_map_grant_ref *kmap_op, unsigned long
> > > > > pfn)
> > > > 
> > > > Do we really need to add another additional parameter to
> > > > m2p_add_override?
> > > > I would just let m2p_add_override and m2p_remove_override call
> > > > page_to_pfn again. It is not that expensive.
> > > Yes, because that page_to_pfn can return something different. That's why
> > > the
> > > v2 patches failed.
> > 
> > I am really curious: how can page_to_pfn return something different?
> > I don't think is supposed to happen.
> You call set_phys_to_machine before calling m2p* functions.

set_phys_to_machine changes the physical to machine mapping, that would
be the mfn corresponding to a given pfn. It shouldn't affect the output
of page_to_pfn that returns the pfn corresponding to a given struct
page. The calculation of which is based on address offsets and should be
static and unaffected by things like set_phys_to_machine.

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