[xen staging] x86/p2m: avoid unnecessary calls of write_p2m_entry_pre() hook

[patchbot@xxxxxxx]

commit 570065b9d5a8a00c5c1aab9e13e9e1ad3e35ce7d
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Jan 8 16:49:23 2021 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Jan 8 16:49:23 2021 +0100

    x86/p2m: avoid unnecessary calls of write_p2m_entry_pre() hook
    
    When shattering a large page, we first construct the new page table page
    and only then hook it up. The "pre" hook in this case does nothing, for
    the page starting out all blank. Avoid 512 calls into shadow code in
    this case by passing in INVALID_GFN, indicating the page being updated
    is (not yet) associated with any GFN. (The alternative to this change
    would be to actually pass in a correct GFN, which can't be all the same
    on every loop iteration.)
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/mm/p2m-pt.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/mm/p2m-pt.c b/xen/arch/x86/mm/p2m-pt.c
index 5fa0d30ce7..848773e1cd 100644
--- a/xen/arch/x86/mm/p2m-pt.c
+++ b/xen/arch/x86/mm/p2m-pt.c
@@ -134,7 +134,7 @@ static int write_p2m_entry(struct p2m_domain *p2m, unsigned 
long gfn,
 
         paging_lock(d);
 
-        if ( p2m->write_p2m_entry_pre )
+        if ( p2m->write_p2m_entry_pre && gfn != gfn_x(INVALID_GFN) )
             p2m->write_p2m_entry_pre(d, gfn, p, new, level);
 
         oflags = l1e_get_flags(*p);
@@ -290,7 +290,8 @@ p2m_next_level(struct p2m_domain *p2m, void **table,
         {
             new_entry = l1e_from_pfn(pfn | (i << ((level - 1) * 
PAGETABLE_ORDER)),
                                      flags);
-            rc = write_p2m_entry(p2m, gfn, l1_entry + i, new_entry, level);
+            rc = write_p2m_entry(p2m, gfn_x(INVALID_GFN), l1_entry + i,
+                                 new_entry, level);
             if ( rc )
             {
                 unmap_domain_page(l1_entry);
--
generated by git-patchbot for /home/xen/git/xen.git#staging