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Subject[PATCH 2/2] x86/mm: Document how CR4.PCIDE restore works
Date
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>
---
arch/x86/kernel/cpu/common.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 40cb4d0a5982..4c31a6585333 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -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 top be restored early.
+ */
cr4_set_bits(X86_CR4_PCIDE);
} else {
/*
--
2.13.5
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