We make sure that the stack is always mapped in pin_stack_pages by
simply calling demand_page, but that calls get_user_pages() to find
the pfn, which is way overkill since the page is almost certainly
already mapped. So don't call pin_stack_pages every context switch
(unless genuinely a completely clean context, all the kernel mappings
are kept in sync), and when we do call it, have it check if it needs
to call demand_page().
This speeds guest context switch by 25%:
Before:
Time for one context switch via pipe: 10606 nsec
After:
Time for one context switch via pipe: 7805 nsec
Native:
Time for one context switch via pipe: 4701 nsec
Signed-off-by: Rusty Russell <[EMAIL PROTECTED]>
diff -r 06b3a533da77 arch/i386/lguest/page_tables.c
--- a/arch/i386/lguest/page_tables.c Wed Feb 21 12:20:20 2007 +1100
+++ b/arch/i386/lguest/page_tables.c Wed Feb 21 18:13:00 2007 +1100
@@ -155,14 +158,29 @@ int demand_page(struct lguest *lg, u32 v
return page_in(lg, vaddr, (write ? _PAGE_DIRTY : 0)|_PAGE_ACCESSED);
}
+/* This is much faster than the full demand_page logic. */
+static int page_writable(struct lguest *lg, unsigned long vaddr)
+{
+ u32 *top, *pte;
+
+ top = toplev(lg, lg->pgdidx, vaddr);
+ if (!(*top & _PAGE_PRESENT))
+ return 0;
+
+ pte = pteof(lg, *top, vaddr);
+ return (*pte & (_PAGE_PRESENT|_PAGE_RW)) == (_PAGE_PRESENT|_PAGE_RW);
+}
+
void pin_stack_pages(struct lguest *lg)
{
unsigned int i;
u32 stack = lg->state->tss.esp1;
- for (i = 0; i < lg->stack_pages; i++)
- if (!demand_page(lg, stack - i*PAGE_SIZE, 1))
+ for (i = 0; i < lg->stack_pages; i++) {
+ if (!page_writable(lg, stack - i * PAGE_SIZE)
+ && !demand_page(lg, stack - i * PAGE_SIZE, 1))
kill_guest(lg, "bad stack page [EMAIL PROTECTED]", i,
stack);
+ }
}
static unsigned int find_pgdir(struct lguest *lg, u32 pgtable)
@@ -198,7 +216,7 @@ void guest_pagetable_flush_user(struct l
flush_user_mappings(lg, lg->pgdidx);
}
-static unsigned int new_pgdir(struct lguest *lg, u32 cr3)
+static unsigned int new_pgdir(struct lguest *lg, u32 cr3, int *blank_pgdir)
{
unsigned int next;
@@ -207,6 +225,9 @@ static unsigned int new_pgdir(struct lgu
lg->pgdirs[next].pgdir = (u32 *)get_zeroed_page(GFP_KERNEL);
if (!lg->pgdirs[next].pgdir)
next = lg->pgdidx;
+ else
+ /* There are no mappings: you'll need to re-pin */
+ *blank_pgdir = 1;
}
lg->pgdirs[next].cr3 = cr3;
/* Release all the non-kernel mappings. */
@@ -217,14 +238,15 @@ static unsigned int new_pgdir(struct lgu
void guest_new_pagetable(struct lguest *lg, u32 pgtable)
{
- int newpgdir;
+ int newpgdir, repin = 0;
newpgdir = find_pgdir(lg, pgtable);
if (newpgdir == ARRAY_SIZE(lg->pgdirs))
- newpgdir = new_pgdir(lg, pgtable);
+ newpgdir = new_pgdir(lg, pgtable, &repin);
lg->pgdidx = newpgdir;
lg->cr3 = __pa(lg->pgdirs[lg->pgdidx].pgdir);
- pin_stack_pages(lg);
+ if (repin)
+ pin_stack_pages(lg);
}
static void release_all_pagetables(struct lguest *lg)
-
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