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authorAndy Lutomirski <luto@kernel.org>2017-09-06 19:54:54 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-09-06 20:12:57 -0700
commit1c9fe4409ce3e9c78b1ed96ee8ed699d4f03bf33 (patch)
treeb0a9e9d2d4a7062dc6f20c463109af14e56c9c7e
parent72c0098d92cedb11c7e0151e84918840a4e96b31 (diff)
downloadlinux-1c9fe4409ce3e9c78b1ed96ee8ed699d4f03bf33.tar.gz
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: Andy Lutomirski <luto@kernel.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--arch/x86/kernel/cpu/common.c13
1 files changed, 13 insertions, 0 deletions
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 40cb4d0a5982..fb1d3358a4af 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 to be restored early.
+ */
cr4_set_bits(X86_CR4_PCIDE);
} else {
/*