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Re: [Xen-devel] [PATCH MM-PART3 v2 05/12] xen/arm: mm: Introduce _PAGE_PRESENT and _PAGE_POPULATE



Hi Stefano,

On 11/06/2019 23:35, Stefano Stabellini wrote:
On Tue, 14 May 2019, Julien Grall wrote:
At the moment, the flags are not enough to describe what kind of update
will done on the VA range. They need to be used in conjunction with the
enum xenmap_operation.

It would be more convenient to have all the information for the update
in a single place.

Two new flags are added to remove the relience on xenmap_operation:
     - _PAGE_PRESENT: Indicate whether we are adding/removing the mapping
     - _PAGE_POPULATE: Indicate whether we only populate page-tables

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
Reviewed-by: Andrii Anisov <andrii_anisov@xxxxxxxx>

Looking ahead in this series, I know that this is done so that later on
you can remove enum xenmap_operation. But what is the end goal? Why do
we want to remove enum xenmap_operation? I guess it is to make the code
more reusable?

The end goal is to streamline as much as possible to page-table update. I wanted to have all the information in flags because it is much easier to reason with one variable over two variables.

Furthermore, x86 code allows map_pages_to_xen(...) to destroy mappings but not the underlying page-tables. This is used for instance for the vunmap to avoid re-creating the page-tables afterwards. I have been thinking to introduce similar things on Arm.

Keeping the xenmap_operation would make it awkward to support it because you would have to translate the flags to the actual operations.


---
     Changes in v2:
         - Add Andrii's reviewed-by
---
  xen/arch/arm/mm.c          | 2 +-
  xen/include/asm-arm/page.h | 9 +++++++--
  2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 9de2a1150f..2192dede55 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1083,7 +1083,7 @@ int map_pages_to_xen(unsigned long virt,
int populate_pt_range(unsigned long virt, unsigned long nr_mfns)
  {
-    return xen_pt_update(RESERVE, virt, INVALID_MFN, nr_mfns, 0);
+    return xen_pt_update(RESERVE, virt, INVALID_MFN, nr_mfns, _PAGE_POPULATE);
  }
int destroy_xen_mappings(unsigned long v, unsigned long e)
diff --git a/xen/include/asm-arm/page.h b/xen/include/asm-arm/page.h
index 2bcdb0f1a5..caf2fac1ff 100644
--- a/xen/include/asm-arm/page.h
+++ b/xen/include/asm-arm/page.h
@@ -76,6 +76,8 @@
   *
   * [0:2] Memory Attribute Index
   * [3:4] Permission flags
+ * [5]   Present bit
+ * [6]   Populate page table

[5] is pretty clear. As a nit, I would probably write:

  [5] Page Present

Better alternative, I will update the comment.


because there is no need to say "bit", the [5] means it is a bit.
Otherwise, something like the following:

  [5] Present in memory

but it's unimportant.

It's [6] that we might want to explain a bit better: if we say "Populate
page table" then every time we want the Xen pagetable to be populated we
would need to pass _PAGE_POPULATE, even when the page is present in
memory. Do you see what I mean? I am not entirely sure how to make it
clearer. Maybe:

To be honest, I was not entirely happy with the current comment. But I also wasn't able to find a better one :).


  [6] Only populate page tables
I am happy with this alternative. I will update the comment.

Cheers,

--
Julien Grall

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