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Commit bb4093de authored by Rusty Russell's avatar Rusty Russell
Browse files

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: default avatarRusty Russell <rusty@rustcorp.com.au>
parent bb6f1d9a

arch/x86/lguest/boot.c

+7 −4
Original line number Diff line number Diff line
@@ -531,6 +531,9 @@ static void lguest_write_cr3(unsigned long cr3) 
{

	lguest_data.pgdir = cr3;

	lazy_hcall1(LHCALL_NEW_PGTABLE, cr3);



	/* 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;

}
@@ -703,9 +706,9 @@ static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval) 
 * 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)

{