[MINI-OS PATCH 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>
---
 arch/x86/mm.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm.c b/arch/x86/mm.c
index bdff38fd..3f5c7ea7 100644
--- a/arch/x86/mm.c
+++ b/arch/x86/mm.c
@@ -640,12 +640,17 @@ void change_readonly(bool readonly)
  * return a valid PTE for a given virtual address. If PTE does not exist,
  * allocate page-table pages.
  */
+#define N_PTS 4
+static char early_pt[PAGE_SIZE * N_PTS] __attribute__((aligned(PAGE_SIZE)));
+static unsigned long n_early_pt = N_PTS;
+
 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 )
@@ -664,7 +669,16 @@ 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 )
+    {
+        n_early_pt--;
+        pt_addr = (unsigned long)&early_pt[n_early_pt * PAGE_SIZE];
+    }
+    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