x86/mm: Document how CR4.PCIDE restore works (1c9fe440) · Commits · 戴 / test

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:

Andy Lutomirski <luto@kernel.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

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 {
		/*

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