[v3 06/12] 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.bae@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
---
 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 1d975cadc256..71cc8abe208c 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -510,10 +510,16 @@ 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 = rd_inactive_gsbase();
+	} 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