[PATCH 2/2] x86: Quark: Comment setup_arch for TLB/PGE bugfix

[Bryan O'Donoghue]

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@xxxxxxxxxxxxxxxxx>
---
 arch/x86/kernel/setup.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 41ead8d..235cfd3 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -879,6 +879,15 @@ void __init setup_arch(char **cmdline_p)
 			KERNEL_PGD_PTRS);
 
 	load_cr3(swapper_pg_dir);
+	/*
+	 * Note: Quark X1000 CPUs advertise PGE incorrectly and require
+	 * a cr3 based tlb flush, so the following __flush_tlb_all()
+	 * will not flush anything because the cpu quirk which clears
+	 * X86_FEATURE_PGE has not been invoked yet. Though due to the
+	 * load_cr3() above the TLB has been flushed already. The
+	 * quirk is invoked before subsequent calls to __flush_tlb_all()
+	 * so proper operation is guaranteed.
+	 */
 	__flush_tlb_all();
 #else
 	printk(KERN_INFO "Command line: %s\n", boot_command_line);
-- 
1.9.1

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