x86: don't use large pages to map the first 2/4MB of memory

Intel recommends to not use large pages for the first 1MB
of the physical memory because there are fixed size MTRRs there
which cause splitups in the TLBs.

On AMD doing so is also a good idea.

The implementation is a little different between 32bit and 64bit.
On 32bit I just taught the initial page table set up about this
because it was very simple to do. This also has the advantage
that the risk of a prefetch ever seeing the page even
if it only exists for a short time is minimized.

On 64bit that is not quite possible, so use set_memory_4k() a little
later (in check_bugs) instead.

Signed-off-by: Andi Kleen <ak@suse.de>
Acked-by: andreas.herrmann3@amd.com
Cc: mingo@elte.hu
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 6 insertions, 1 deletions

diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
index fc3ace2e88f1..1500dc8d63e4 100644
--- a/arch/x86/mm/init_32.c
+++ b/arch/x86/mm/init_32.c
@@ -181,8 +181,13 @@ static void __init kernel_physical_mapping_init(pgd_t *pgd_base)
                         /*
                          * Map with big pages if possible, otherwise
                          * create normal page tables:
+                         *
+                         * Don't use a large page for the first 2/4MB of memory
+                         * because there are often fixed size MTRRs in there
+                         * and overlapping MTRRs into large pages can cause
+                         * slowdowns.
                          */
-                        if (cpu_has_pse) {
+                        if (cpu_has_pse && !(pgd_idx == 0 && pmd_idx == 0)) {
                                 unsigned int addr2;
                                 pgprot_t prot = PAGE_KERNEL_LARGE;