diff options
| author | Andy Lutomirski <luto@kernel.org> | 2017-09-06 19:54:54 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2017-09-06 20:12:57 -0700 |
| commit | 1c9fe4409ce3e9c78b1ed96ee8ed699d4f03bf33 (patch) | |
| tree | b0a9e9d2d4a7062dc6f20c463109af14e56c9c7e | |
| parent | 72c0098d92cedb11c7e0151e84918840a4e96b31 (diff) | |
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.c | 13 |
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 { /* |