Re: [PATCH 2/2] xen: rename wrong named pfn related variables

[Juergen Gross <jgross@xxxxxxxx>]

On 03.08.21 12:42, Jan Beulich wrote:
> On 30.07.2021 11:00, Juergen Gross wrote:
> > On 16.06.21 12:43, Juergen Gross wrote:
> > > On 16.06.21 11:56, Jan Beulich wrote:
> > > > On 16.06.2021 09:30, Juergen Gross wrote:
> > > > > --- a/arch/x86/xen/p2m.c
> > > > > +++ b/arch/x86/xen/p2m.c
> > > > > @@ -95,8 +95,8 @@ unsigned long *xen_p2m_addr __read_mostly;
> > > > >    EXPORT_SYMBOL_GPL(xen_p2m_addr);
> > > > >    unsigned long xen_p2m_size __read_mostly;
> > > > >    EXPORT_SYMBOL_GPL(xen_p2m_size);
> > > > > -unsigned long xen_max_p2m_pfn __read_mostly;
> > > > > -EXPORT_SYMBOL_GPL(xen_max_p2m_pfn);
> > > > > +unsigned long xen_p2m_max_size __read_mostly;
> > > > > +EXPORT_SYMBOL_GPL(xen_p2m_max_size);
> > > > Instead of renaming the exported variable (which will break consumers
> > > > anyway), how about dropping the apparently unneeded export at this
> > > > occasion?
> > > Why do you think it isn't needed? It is being referenced via the inline
> > > function __pfn_to_mfn() in arch/x86/include/asm/xen/page.h. And
> > > __pfn_to_mfn() is used via lots of other inline functions and macros.
> > >
> > > > Further it looks to me as if xen_p2m_size and this variable
> > > > were actually always kept in sync, so I'd like to put up the question
> > > > of dropping one of the two.
> > > Hmm, should be possible, yes.
> > Looking into this it seems this is not possible.
> >
> > xen_p2m_size always holds the number of p2m entries in the p2m table,
> > including invalid ones at the end. xen_p2m_pfn_limit however contains
> > the (rounded up) index after the last valid p2m entry.
> I'm afraid I can't follow:
>
> xen_build_dynamic_phys_to_machine() sets xen_p2m_size and then syncs
> its value to what so far has been xen_max_p2m_pfn.
>
> xen_vmalloc_p2m_tree() sets xen_max_p2m_pfn and then syncs its value
> to xen_p2m_size.
>
> I therefore can't see how the two values would hold different values,
> except for the brief periods between updating one and then the other.
The brief period in xen_vmalloc_p2m_tree() is the problematic one. The
different values are especially important for the calls of
__pfn_to_mfn() during xen_rebuild_p2m_list().


Juergen

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