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



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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