lguest: restore boot speed
lguest is dumb and drops *all* the pagetables for set_pte (which is
only used for kernel mapping manipulation, so it's OK without highmem).
But it's used a lot in boot, too. As a guest optimization, we
suppressed this flushing until the first page switch. Now we have
initial_page_table, that happens much earlier, so extend the heuristic
to wait until we switch to something other than the swapper_pg_dir or
initial_page_table.
As measured on my laptop under kvm, this dropped the time-to-mount-root
from 48 seconds to 4.3 seconds.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index 45e64b3..24e4973 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -531,7 +531,10 @@
{
lguest_data.pgdir = cr3;
lazy_hcall1(LHCALL_NEW_PGTABLE, cr3);
- cr3_changed = true;
+
+ /* These two page tables are simple, linear, and used during boot */
+ if (cr3 != __pa(swapper_pg_dir) && cr3 != __pa(initial_page_table))
+ cr3_changed = true;
}
static unsigned long lguest_read_cr3(void)
@@ -703,9 +706,9 @@
* to forget all of them. Fortunately, this is very rare.
*
* ... except in early boot when the kernel sets up the initial pagetables,
- * which makes booting astonishingly slow: 1.83 seconds! So we don't even tell
- * the Host anything changed until we've done the first page table switch,
- * which brings boot back to 0.25 seconds.
+ * which makes booting astonishingly slow: 48 seconds! So we don't even tell
+ * the Host anything changed until we've done the first real page table switch,
+ * which brings boot back to 4.3 seconds.
*/
static void lguest_set_pte(pte_t *ptep, pte_t pteval)
{