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Message-ID: <20240807054722.682375-4-xin@zytor.com>
Date: Tue, 6 Aug 2024 22:47:22 -0700
From: "Xin Li (Intel)" <xin@...or.com>
To: linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
peterz@...radead.org, andrew.cooper3@...rix.com, seanjc@...gle.com
Subject: [PATCH v1 3/3] x86/entry: Set FRED RSP0 on return to userspace instead of context switch
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@...gle.com>
Signed-off-by: Xin Li (Intel) <xin@...or.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
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