Re: [Qemu-devel] [PATCH] exec: Skip mru section if it's a partial page and not resolving subpage

[BALATON Zoltan]

On Wed, 15 Nov 2017, Paolo Bonzini wrote:
This is another possibility:
diff --git a/exec.c b/exec.c
index 97a24a875e..3bb9fcf257 100644
--- a/exec.c
+++ b/exec.c
@@ -410,22 +410,16 @@ static MemoryRegionSection 
*address_space_lookup_region(AddressSpaceDispatch *d,
{
    MemoryRegionSection *section = atomic_read(&d->mru_section);
    subpage_t *subpage;
-    bool update;

-    if (section && section != &d->map.sections[PHYS_SECTION_UNASSIGNED] &&
-        section_covers_addr(section, addr)) {
-        update = false;
-    } else {
+    if (!section || section == &d->map.sections[PHYS_SECTION_UNASSIGNED] ||
+        !section_covers_addr(section, addr)) {
        section = phys_page_find(d, addr);
-        update = true;
+        atomic_set(&d->mru_section, section);
    }
    if (resolve_subpage && section->mr->subpage) {
        subpage = container_of(section->mr, subpage_t, iomem);
        section = &d->map.sections[subpage->sub_section[SUBPAGE_IDX(addr)]];
    }
-    if (update) {
-        atomic_set(&d->mru_section, section);
-    }
    return section;
}


It will skip the expensive phys_page_find but not the cheap subpage lookup.
Does it work for you?

This also works but I can't understand why becuase the problematic call to 
this function which got the wrong result from section_covers_addr was with 
resolve_subpage=false and in that case this looks identical to the 
original which did not work. Unless this function is called multiple times 
and another call with resolve_subpage=true now somehow works better with 
this change or replacing the mru_section earlier before subpage lookup 
makes a difference which is not obvious from this function.

Anyway, it fixes the problem I see so you can use this instead of my 
patch.

Thank you,
BALATON Zoltan