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Commit 1c9fe440 authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Linus Torvalds
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x86/mm: Document how CR4.PCIDE restore works 



While debugging a problem, I thought that using
cr4_set_bits_and_update_boot() to restore CR4.PCIDE would be
helpful.  It turns out to be counterproductive.

Add a comment documenting how this works.

Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 72c0098d

arch/x86/kernel/cpu/common.c

+13 −0
Original line number Diff line number Diff line
@@ -333,6 +333,19 @@ static void setup_pcid(struct cpuinfo_x86 *c) 
{

	if (cpu_has(c, X86_FEATURE_PCID)) {

		if (cpu_has(c, X86_FEATURE_PGE)) {

			/*

			 * We'd like to use cr4_set_bits_and_update_boot(),

			 * but we can't.  CR4.PCIDE is special and can only

			 * be set in long mode, and the early CPU init code

			 * doesn't know this and would try to restore CR4.PCIDE

			 * prior to entering long mode.

			 *

			 * Instead, we rely on the fact that hotplug, resume,

			 * etc all fully restore CR4 before they write anything

			 * that could have nonzero PCID bits to CR3.  CR4.PCIDE

			 * has no effect on the page tables themselves, so we

			 * don't need it to be restored early.

			 */

			cr4_set_bits(X86_CR4_PCIDE);

		} else {

			/*