[MINI-OS PATCH v2 1/2] mm: provide a way to do very early page table allocations

[Juergen Gross <jgross@xxxxxxxx>]

Add a small pool of statically allocated memory pages to be handed out
for very early page table allocations.

This will make it possible to do virtual allocations e.g. for mapping
the shared info page.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V2:
- add comment (Jan Beulich)
- drop N_PT in favor of ARRAY_SIZE() (Jan Beulich)
- use 2-dimensional array (Jan Beulich)
---
 arch/x86/mm.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm.c b/arch/x86/mm.c
index ebc85e0d..fd808884 100644
--- a/arch/x86/mm.c
+++ b/arch/x86/mm.c
@@ -640,13 +640,20 @@ void change_readonly(bool readonly)
 /*
  * return a valid PTE for a given virtual address. If PTE does not exist,
  * allocate page-table pages.
+ * Provide a small pool for allocating some page tables very early. Those are
+ * needed currently for mapping the shared info page, so 2 pages ought to be
+ * enough. Add one spare page anyway.
  */
+static char early_pt[4][PAGE_SIZE] __attribute__((aligned(PAGE_SIZE)));
+static unsigned int n_early_pt = ARRAY_SIZE(early_pt);
+
 static int need_pgt_func(unsigned long va, unsigned int lvl, bool is_leaf,
                          pgentry_t *pte, void *par)
 {
     pgentry_t **result = par;
     unsigned long pt_mfn;
     unsigned long pt_pfn;
+    unsigned long pt_addr;
     unsigned int idx;
 
     if ( !is_leaf )
@@ -665,7 +672,11 @@ static int need_pgt_func(unsigned long va, unsigned int 
lvl, bool is_leaf,
     }
 
     pt_mfn = virt_to_mfn(pte);
-    pt_pfn = virt_to_pfn(alloc_page());
+    if ( n_early_pt )
+        pt_addr = (unsigned long)early_pt[--n_early_pt];
+    else
+        pt_addr = alloc_page();
+    pt_pfn = virt_to_pfn(pt_addr);
     if ( !pt_pfn )
         return -1;
     idx = idx_from_va_lvl(va, lvl);
-- 
2.43.0