[PATCH V7 01/11] xen/pci: arm: add stub for is_memory_hole

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>

Add a stub for is_memory_hole which is required for PCI passthrough
on Arm.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
---
OT: It looks like the discussion got stuck. As I understand this
patch is not immediately needed in the context of current series
as PCI passthrough is not enabled on Arm at the moment. So the patch
could be added later on, but it is needed to allow PCI passthrough
to be built on Arm for those who want to test it.

Copy here some context provided by Julien:

Here a summary of the discussion (+ some my follow-up thoughts):

is_memory_hole() was recently introduced on x86 (see commit 75cc460a1b8c
"xen/pci: detect when BARs are not suitably positioned") to check
whether the BAR are positioned outside of a valid memory range. This was
introduced to work-around quirky firmware.

In theory, this could also happen on Arm. In practice, this may not
happen but it sounds better to sanity check that the BAR contains
"valid" I/O range.

On x86, this is implemented by checking the region is not described is
in the e820. IIUC, on Arm, the BARs have to be positioned in pre-defined
ranges. So I think it would be possible to implement is_memory_hole() by
going through the list of hostbridges and check the ranges.

But first, I'd like to confirm my understanding with Rahul, and others.

If we were going to go this route, I would also rename the function to
be better match what it is doing (i.e. it checks the BAR is correctly
placed). As a potentially optimization/hardening for Arm, we could pass
the hostbridge so we don't have to walk all of them.
---
 xen/arch/arm/mm.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 009b8cd9ef..bb34b97eb5 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1708,6 +1708,12 @@ unsigned long get_upper_mfn_bound(void)
     return max_page - 1;
 }
 
+bool is_memory_hole(mfn_t start, mfn_t end)
+{
+    /* TODO: this needs to be properly implemented. */
+    return true;
+}
+
 /*
  * Local variables:
  * mode: C
-- 
2.25.1