[PATCH v3 8/8] x86/mm: adjust loop in arch_init_memory() to iterate over the PDX space

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

There's a loop in arch_init_memory() that iterates over holes and non-RAM
regions to possibly mark any page_info structures matching those addresses
as IO.  The looping there is done over the PFN space.

PFNs not covered by the PDX space will always fail the mfn_valid() check,
hence re-write the loop to iterate over the PDX space and avoid checking
any holes that are not covered by the PDX translation.

On a system with a ~6TiB hole this change together with using PDX
compression reduces boot time in approximately 20 seconds.  Xen boot time
without the change is ~50s, with the change it's ~30s.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v2:
 - New in this version.
---
 xen/arch/x86/mm.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index e7fd56c7ce90..647bf0b41db6 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -275,7 +275,7 @@ static void __init assign_io_page(struct page_info *page)
 
 void __init arch_init_memory(void)
 {
-    unsigned long i, pfn, rstart_pfn, rend_pfn, iostart_pfn, ioend_pfn;
+    unsigned long i, pfn, rstart_pfn, rend_pfn, iostart_pfn, ioend_pfn, pdx;
 
     /*
      * Basic guest-accessible flags:
@@ -328,9 +328,14 @@ void __init arch_init_memory(void)
             destroy_xen_mappings((unsigned long)mfn_to_virt(iostart_pfn),
                                  (unsigned long)mfn_to_virt(ioend_pfn));
 
-        /* Mark as I/O up to next RAM region. */
-        for ( ; pfn < rstart_pfn; pfn++ )
+        /*
+         * Mark as I/O up to next RAM region.  Iterate over the PDX space to
+         * skip holes which would always fail the mfn_valid() check.
+         */
+        for ( pdx = pfn_to_pdx(pfn); pdx < pfn_to_pdx(rstart_pfn); pdx++ )
         {
+            pfn = pdx_to_pfn(pdx);
+
             if ( !mfn_valid(_mfn(pfn)) )
                 continue;
 
-- 
2.49.0