[PATCH 3/3] x86: Quark: Comment setup_arch for TLB/PGE bugfix

["Bryan O'Donoghue" <pure.logic@nexus-software.ie>]

Quark X1000 requires CR3 to be rewritten to flush TLB entries
irrespective of the PGE bits in CR4 or PTE.PGE

Add a comment to setup_arch to indicate that the code

load_cr3(swapper_pg_dir);
__flush_tlb_all();

Will already have flushed the TLB @ the CR3 reload allowing us
to skip over a potential if/else for Quark.

This comment clearly states the bug, the behaviour we rely on
and the reason why we only do it this way - one time.

Later on cpu_has_pge() will be false due to a fixup in
intel_init_early() and __flush_tlb_all() will work as expected
from that point onwards

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
---
 arch/x86/kernel/setup.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 41ead8d..98b6660 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -878,6 +878,18 @@ void __init setup_arch(char **cmdline_p)
 			initial_page_table + KERNEL_PGD_BOUNDARY,
 			KERNEL_PGD_PTRS);
 
+	/*
+	 * Locate the page directory and flush the TLB.
+	 *
+	 * On Quark X1000 CPUs we still have the PGE bit incorrectly set
+	 * due to a processor erratum, so __flush_tlb_all() is not yet
+	 * doing what it says.  Fortunately we have a cr3 load here,
+	 * which is what is needed on this processor to flush TLBs, so
+	 * there's no need to add a Quark X1000 quirk here.
+	 *
+	 * early_init_intel will unset the X86_FEATURE_PGE flag later
+	 * and __flush_tlb_all() will flush via cr3
+	 */
 	load_cr3(swapper_pg_dir);
 	__flush_tlb_all();
 #else
-- 
1.9.1