Re: [PATCH V3 6/8] xen: Hide get_phys_to_machine() to be able to tune common path

[Juergen Gross]

On 11/12/2014 11:18 PM, Konrad Rzeszutek Wilk wrote:
> On Tue, Nov 11, 2014 at 06:43:44AM +0100, Juergen Gross wrote:
> > Today get_phys_to_machine() is always called when the mfn for a pfn
> > is to be obtained. Add a wrapper __pfn_to_mfn() as inline function
> > to be able to avoid calling get_phys_to_machine() when possible as
>
> s/when/where/

No. It's not a matter of the caller, but of the p2m list entry.

> > soon as the switch to a linear mapped p2m list has been done.
>
> But your inline function still calls get_phys_to_machine?

Sure. The switch is done in the next patch. David asked me to split
the patch doing the preparation by adding __pfn_tom_mfn() in an own
patch.

> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> >   arch/x86/include/asm/xen/page.h | 27 +++++++++++++++++++++------
> >   arch/x86/xen/mmu.c              |  2 +-
> >   arch/x86/xen/p2m.c              |  6 +++---
> >   3 files changed, 25 insertions(+), 10 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
> > index 28fa795..07d8a7b 100644
> > --- a/arch/x86/include/asm/xen/page.h
> > +++ b/arch/x86/include/asm/xen/page.h
> > @@ -59,6 +59,22 @@ extern int clear_foreign_p2m_mapping(struct gnttab_unmap_grant_ref *unmap_ops,
> >   				     struct page **pages, unsigned int count);
> >   extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long pfn);
> >
> > +/*
> > + * When to use pfn_to_mfn(), __pfn_to_mfn() or get_phys_to_machine():
> > + * - pfn_to_mfn() returns either INVALID_P2M_ENTRY or the mfn. In case of an
> > + *   identity entry the identity indicator will be cleared.
>
> Why don't you say : In case of identity PFN the same PFN is returned.
>
> But you did miss that also the FOREIGN_FRAME_BIT is cleared.

I'll reword the comment.

> > + * - __pfn_to_mfn() returns the found entry of the p2m table. A possibly set
>
> s/of the/in the/
> > + *   identity indicator will be still set. __pfn_to_mfn() is encapsulating
> .. be still set if the PFN is an identity one.
> > + *   get_phys_to_machine() and might skip that function if possible to speed
> > + *   up the common path.
>
> How is is skipping that function? The patch below does no such thing?

The next patch in this series does.

> > + * - get_phys_to_machine() is basically the same as __pfn_to_mfn(), but
> > + *   without any short cuts for the common fast path.
>
> Right. Perhpas we should call it 'slow_p2m' instead of the 'get_phys_to_machine'.

That's a matter of taste, I think. I can change it if nobody else
objects.


Juergen
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