[PATCH v5 07/13] x86/fsgsbase/64: When copying a thread, use the FSGSBASE instructions if available

[Chang S. Bae]

Copy real FS/GSBASE values instead of approximation when FSGSBASE is
enabled.

Factoring out to save_fsgs() does not result in the same behavior because
save_base_legacy() does not copy FS/GSBASE when the index is zero.

Signed-off-by: Chang S. Bae <chang.seok....@intel.com>
Cc: Andy Lutomirski <l...@kernel.org>
Cc: H. Peter Anvin <h...@zytor.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: Andi Kleen <a...@linux.intel.com>
---
 arch/x86/kernel/process_64.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index d8ade9530fdb..648e43b58c69 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -477,10 +477,16 @@ int copy_thread_tls(unsigned long clone_flags, unsigned 
long sp,
        p->thread.sp = (unsigned long) fork_frame;
        p->thread.io_bitmap_ptr = NULL;
 
-       savesegment(gs, p->thread.gsindex);
-       p->thread.gsbase = p->thread.gsindex ? 0 : me->thread.gsbase;
        savesegment(fs, p->thread.fsindex);
-       p->thread.fsbase = p->thread.fsindex ? 0 : me->thread.fsbase;
+       savesegment(gs, p->thread.gsindex);
+       if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
+               p->thread.fsbase = rdfsbase();
+               p->thread.gsbase = __rdgsbase_inactive();
+       } else {
+               /* save_base_legacy() does not set base when index is zero. */
+               p->thread.fsbase = p->thread.fsindex ? 0 : me->thread.fsbase;
+               p->thread.gsbase = p->thread.gsindex ? 0 : me->thread.gsbase;
+       }
        savesegment(es, p->thread.es);
        savesegment(ds, p->thread.ds);
        memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));
-- 
2.19.1