[PATCH v1 3/3] x86/entry: Set FRED RSP0 on return to userspace instead of context switch

["Xin Li (Intel)" <xin@zytor.com>]

FRED RSP0 is a per task constant pointing to top of its kernel stack
for user level event delivery, and needs to be updated when a task is
scheduled in.

Introduce a new TI flag TIF_LOAD_USER_STATES to track whether FRED RSP0
needs to be loaded, and do the actual load of FRED RSP0 in
arch_exit_to_user_mode_prepare() if the TI flag is set, thus to avoid a
fair number of WRMSRs in both KVM and the kernel.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
---
 arch/x86/include/asm/entry-common.h | 5 +++++
 arch/x86/include/asm/switch_to.h    | 3 +--
 arch/x86/include/asm/thread_info.h  | 2 ++
 arch/x86/kernel/cpu/cpuid-deps.c    | 1 -
 4 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 4c78b99060b5..ae365579efb3 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -51,6 +51,11 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 		if (ti_work & _TIF_USER_RETURN_NOTIFY)
 			fire_user_return_notifiers();
 
+		if (cpu_feature_enabled(X86_FEATURE_FRED) &&
+		    (ti_work & _TIF_LOAD_USER_STATES))
+			wrmsrns(MSR_IA32_FRED_RSP0,
+				(unsigned long)task_stack_page(current) + THREAD_SIZE);
+
 		if (unlikely(ti_work & _TIF_IO_BITMAP))
 			tss_update_io_bitmap();
 
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index c3bd0c0758c9..a31ea544cc0e 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -71,8 +71,7 @@ static inline void update_task_stack(struct task_struct *task)
 	this_cpu_write(cpu_tss_rw.x86_tss.sp1, task->thread.sp0);
 #else
 	if (cpu_feature_enabled(X86_FEATURE_FRED)) {
-		/* WRMSRNS is a baseline feature for FRED. */
-		wrmsrns(MSR_IA32_FRED_RSP0, (unsigned long)task_stack_page(task) + THREAD_SIZE);
+		set_thread_flag(TIF_LOAD_USER_STATES);
 	} else if (cpu_feature_enabled(X86_FEATURE_XENPV)) {
 		/* Xen PV enters the kernel on the thread stack. */
 		load_sp0(task_top_of_stack(task));
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index 12da7dfd5ef1..fb51904651c0 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -106,6 +106,7 @@ struct thread_info {
 #define TIF_BLOCKSTEP		25	/* set when we want DEBUGCTLMSR_BTF */
 #define TIF_LAZY_MMU_UPDATES	27	/* task is updating the mmu lazily */
 #define TIF_ADDR32		29	/* 32-bit address space on 64 bits */
+#define TIF_LOAD_USER_STATES	30	/* Load user level states */
 
 #define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
@@ -128,6 +129,7 @@ struct thread_info {
 #define _TIF_BLOCKSTEP		(1 << TIF_BLOCKSTEP)
 #define _TIF_LAZY_MMU_UPDATES	(1 << TIF_LAZY_MMU_UPDATES)
 #define _TIF_ADDR32		(1 << TIF_ADDR32)
+#define _TIF_LOAD_USER_STATES	(1 << TIF_LOAD_USER_STATES)
 
 /* flags to check in __switch_to() */
 #define _TIF_WORK_CTXSW_BASE					\
diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index b7d9f530ae16..8bd84114c2d9 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -83,7 +83,6 @@ static const struct cpuid_dep cpuid_deps[] = {
 	{ X86_FEATURE_AMX_TILE,			X86_FEATURE_XFD       },
 	{ X86_FEATURE_SHSTK,			X86_FEATURE_XSAVES    },
 	{ X86_FEATURE_FRED,			X86_FEATURE_LKGS      },
-	{ X86_FEATURE_FRED,			X86_FEATURE_WRMSRNS   },
 	{}
 };
 
-- 
2.45.2