[tip:x86/fpu] x86/entry: Add TIF_NEED_FPU_LOAD

[tip-bot for Sebastian Andrzej Siewior]

Commit-ID:  383c252545edcc708128e2028a2318b05c45ede4
Gitweb:     https://git.kernel.org/tip/383c252545edcc708128e2028a2318b05c45ede4
Author:     Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
AuthorDate: Wed, 3 Apr 2019 18:41:45 +0200
Committer:  Borislav Petkov <bp@xxxxxxx>
CommitDate: Thu, 11 Apr 2019 16:21:51 +0200

x86/entry: Add TIF_NEED_FPU_LOAD

Add TIF_NEED_FPU_LOAD. This flag is used for loading the FPU registers
before returning to userland. It must not be set on systems without a
FPU.

If this flag is cleared, the CPU's FPU registers hold the latest,
up-to-date content of the current task's (current()) FPU registers.
The in-memory copy (union fpregs_state) is not valid.

If this flag is set, then all of CPU's FPU registers may hold a random
value (except for PKRU) and it is required to load the content of the
FPU registers on return to userland.

Introduce it now as a preparatory change before adding the main feature.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Aubrey Li <aubrey.li@xxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: "Jason A. Donenfeld" <Jason@xxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: kvm ML <kvm@xxxxxxxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: x86-ml <x86@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190403164156.19645-17-bigeasy@xxxxxxxxxxxxx
---
 arch/x86/include/asm/fpu/internal.h | 8 ++++++++
 arch/x86/include/asm/thread_info.h  | 2 ++
 2 files changed, 10 insertions(+)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 6eb4a0b1ad0e..da75d7b3e37d 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -508,6 +508,14 @@ static inline void __fpregs_load_activate(struct fpu *fpu, int cpu)
  *  - switch_fpu_finish() restores the new state as
  *    necessary.
  *
+ * If TIF_NEED_FPU_LOAD is cleared then the CPU's FPU registers
+ * are saved in the current thread's FPU register state.
+ *
+ * If TIF_NEED_FPU_LOAD is set then CPU's FPU registers may not
+ * hold current()'s FPU registers. It is required to load the
+ * registers before returning to userland or using the content
+ * otherwise.
+ *
  * The FPU context is only stored/restored for a user task and
  * ->mm is used to distinguish between kernel and user threads.
  */
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index e0eccbcb8447..f9453536f9bb 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -88,6 +88,7 @@ struct thread_info {
 #define TIF_USER_RETURN_NOTIFY	11	/* notify kernel of userspace return */
 #define TIF_UPROBE		12	/* breakpointed or singlestepping */
 #define TIF_PATCH_PENDING	13	/* pending live patching update */
+#define TIF_NEED_FPU_LOAD	14	/* load FPU on return to userspace */
 #define TIF_NOCPUID		15	/* CPUID is not accessible in userland */
 #define TIF_NOTSC		16	/* TSC is not accessible in userland */
 #define TIF_IA32		17	/* IA32 compatibility process */
@@ -117,6 +118,7 @@ struct thread_info {
 #define _TIF_USER_RETURN_NOTIFY	(1 << TIF_USER_RETURN_NOTIFY)
 #define _TIF_UPROBE		(1 << TIF_UPROBE)
 #define _TIF_PATCH_PENDING	(1 << TIF_PATCH_PENDING)
+#define _TIF_NEED_FPU_LOAD	(1 << TIF_NEED_FPU_LOAD)
 #define _TIF_NOCPUID		(1 << TIF_NOCPUID)
 #define _TIF_NOTSC		(1 << TIF_NOTSC)
 #define _TIF_IA32		(1 << TIF_IA32)