[mini-os master] mm: provide a way to do very early page table allocations

[patchbot@xxxxxxx]

commit ba260dc134aa360da719e1932af05a743a3db18f
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Tue Jul 29 11:22:53 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jul 29 11:22:53 2025 +0200

    mm: provide a way to do very early page table allocations
    
    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>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 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 ebc85e0..995508c 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 3 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);
--
generated by git-patchbot for /home/xen/git/mini-os.git#master