Re: [PATCH 4/5] x86/HAP: move nested-P2M flush calculations out of locked region

[Roger Pau Monné]

On Wed, Oct 28, 2020 at 10:24:12AM +0100, Jan Beulich wrote:
> By latching the old MFN into a local variable, these calculations don't
> depend on anything but local variables anymore. Hence the point in time
> when they get performed doesn't matter anymore, so they can be moved
> past the locked region.
> 
> Signed-off-by: Jan Beulich 

Acked-by: Roger Pau Monné 

Thanks, Roger.

[PATCH 4/5] x86/HAP: move nested-P2M flush calculations out of locked region

[Jan Beulich]

By latching the old MFN into a local variable, these calculations don't
depend on anything but local variables anymore. Hence the point in time
when they get performed doesn't matter anymore, so they can be moved
past the locked region.

Signed-off-by: Jan Beulich 

--- a/xen/arch/x86/mm/hap/hap.c
+++ b/xen/arch/x86/mm/hap/hap.c
@@ -780,7 +780,7 @@ hap_write_p2m_entry(struct p2m_domain *p
 {
 struct domain *d = p2m->domain;
 uint32_t old_flags;
-bool_t flush_nestedp2m = 0;
+mfn_t omfn;
 int rc;
 
 /* We know always use the host p2m here, regardless if the vcpu
@@ -790,21 +790,11 @@ hap_write_p2m_entry(struct p2m_domain *p
 
 paging_lock(d);
 old_flags = l1e_get_flags(*p);
-
-if ( nestedhvm_enabled(d) && (old_flags & _PAGE_PRESENT) 
- && !p2m_get_hostp2m(d)->defer_nested_flush ) {
-/* We are replacing a valid entry so we need to flush nested p2ms,
- * unless the only change is an increase in access rights. */
-mfn_t omfn = l1e_get_mfn(*p);
-mfn_t nmfn = l1e_get_mfn(new);
-
-flush_nestedp2m = !(mfn_eq(omfn, nmfn)
-&& perms_strictly_increased(old_flags, l1e_get_flags(new)) );
-}
+omfn = l1e_get_mfn(*p);
 
 rc = p2m_entry_modify(p2m, p2m_flags_to_type(l1e_get_flags(new)),
   p2m_flags_to_type(old_flags), l1e_get_mfn(new),
-  l1e_get_mfn(*p), level);
+  omfn, level);
 if ( rc )
 {
 paging_unlock(d);
@@ -817,7 +807,14 @@ hap_write_p2m_entry(struct p2m_domain *p
 
 paging_unlock(d);
 
-if ( flush_nestedp2m )
+if ( nestedhvm_enabled(d) && (old_flags & _PAGE_PRESENT) &&
+ !p2m_get_hostp2m(d)->defer_nested_flush &&
+ /*
+  * We are replacing a valid entry so we need to flush nested p2ms,
+  * unless the only change is an increase in access rights.
+  */
+ (!mfn_eq(omfn, l1e_get_mfn(new)) ||
+  !perms_strictly_increased(old_flags, l1e_get_flags(new))) )
 p2m_flush_nestedp2m(d);
 
 return 0;