Re: [PATCH 3/3] x86/fpu: Remove init_task FPU state dependencies, add debugging warning
From: Ning, Hongyu
Date: Wed Jun 26 2024 - 23:51:11 EST
On 2024/6/24 14:47, Ning, Hongyu wrote:
On 2024/6/5 16:35, Ingo Molnar wrote:
init_task's FPU state initialization was a bit of a hack:
__x86_init_fpu_begin = .;
. = __x86_init_fpu_begin + 128*PAGE_SIZE;
__x86_init_fpu_end = .;
But the init task isn't supposed to be using the FPU in any case,
so remove the hack and add in some debug warnings.
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Uros Bizjak <ubizjak@xxxxxxxxx>
Link: https://lore.kernel.org/r/ZgaNs1lC2Y+AnRG4@xxxxxxxxx
---
arch/x86/include/asm/processor.h | 6 +++++-
arch/x86/kernel/fpu/core.c | 12 +++++++++---
arch/x86/kernel/fpu/init.c | 5 ++---
arch/x86/kernel/fpu/xstate.c | 3 ---
arch/x86/kernel/vmlinux.lds.S | 4 ----
5 files changed, 16 insertions(+), 14 deletions(-)
diff --git a/arch/x86/include/asm/processor.h
b/arch/x86/include/asm/processor.h
index 249c5fa20de4..ed8981866f4d 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -504,7 +504,11 @@ struct thread_struct {
#endif
};
-#define x86_task_fpu(task) ((struct fpu *)((void *)task +
sizeof(*task)))
+#ifdef CONFIG_X86_DEBUG_FPU
+extern struct fpu *x86_task_fpu(struct task_struct *task);
+#else
+# define x86_task_fpu(task) ((struct fpu *)((void *)task +
sizeof(*task)))
+#endif
/*
* X86 doesn't need any embedded-FPU-struct quirks:
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 0ccabcd3bf62..fdc3b227800d 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -51,6 +51,15 @@ static DEFINE_PER_CPU(bool, in_kernel_fpu);
*/
DEFINE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);
+#ifdef CONFIG_X86_DEBUG_FPU
+struct fpu *x86_task_fpu(struct task_struct *task)
+{
+ WARN_ON_ONCE(task == &init_task);
+
+ return (void *)task + sizeof(*task);
+}
+#endif
+
/*
* Can we use the FPU in kernel mode with the
* whole "kernel_fpu_begin/end()" sequence?
@@ -591,10 +600,8 @@ int fpu_clone(struct task_struct *dst, unsigned
long clone_flags, bool minimal,
* This is safe because task_struct size is a multiple of
cacheline size.
*/
struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
- struct fpu *src_fpu = x86_task_fpu(current);
BUILD_BUG_ON(sizeof(*dst) % SMP_CACHE_BYTES != 0);
- BUG_ON(!src_fpu);
/* The new task's FPU state cannot be valid in the hardware. */
dst_fpu->last_cpu = -1;
@@ -657,7 +664,6 @@ int fpu_clone(struct task_struct *dst, unsigned
long clone_flags, bool minimal,
if (update_fpu_shstk(dst, ssp))
return 1;
- trace_x86_fpu_copy_src(src_fpu);
trace_x86_fpu_copy_dst(dst_fpu);
return 0;
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index 11aa31410df2..53580e59e5db 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -38,7 +38,7 @@ static void fpu__init_cpu_generic(void)
/* Flush out any pending x87 state: */
#ifdef CONFIG_MATH_EMULATION
if (!boot_cpu_has(X86_FEATURE_FPU))
- fpstate_init_soft(&x86_task_fpu(current)->fpstate->regs.soft);
+ ;
else
#endif
asm volatile ("fninit");
@@ -164,7 +164,7 @@ static void __init fpu__init_task_struct_size(void)
* Subtract off the static size of the register state.
* It potentially has a bunch of padding.
*/
- task_size -= sizeof(x86_task_fpu(current)->__fpstate.regs);
+ task_size -= sizeof(union fpregs_state);
/*
* Add back the dynamically-calculated register state
@@ -209,7 +209,6 @@ static void __init
fpu__init_system_xstate_size_legacy(void)
fpu_kernel_cfg.default_size = size;
fpu_user_cfg.max_size = size;
fpu_user_cfg.default_size = size;
- fpstate_reset(x86_task_fpu(current));
}
/*
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 90b11671e943..1f37da22ddbe 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -844,9 +844,6 @@ void __init fpu__init_system_xstate(unsigned int
legacy_size)
if (err)
goto out_disable;
- /* Reset the state for the current task */
- fpstate_reset(x86_task_fpu(current));
-
/*
* Update info used for ptrace frames; use standard-format size
and no
* supervisor xstates:
diff --git a/arch/x86/kernel/vmlinux.lds.S
b/arch/x86/kernel/vmlinux.lds.S
index 226244a894da..3509afc6a672 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -170,10 +170,6 @@ SECTIONS
/* equivalent to task_pt_regs(&init_task) */
__top_init_kernel_stack = __end_init_stack -
TOP_OF_KERNEL_STACK_PADDING - PTREGS_SIZE;
- __x86_init_fpu_begin = .;
- . = __x86_init_fpu_begin + 128*PAGE_SIZE;
- __x86_init_fpu_end = .;
-
#ifdef CONFIG_X86_32
/* 32 bit has nosave before _edata */
NOSAVE_DATA
Hi,
we've hit x86/fpu related WARNING and NULL pointer issue during KVM/QEMU
VM booting with latest linux-next kernel, bisect results show it's
related to this commit, would you take a look?
detailed description in https://bugzilla.kernel.org/show_bug.cgi?id=218980
add a quick update:
1. CONFIG_X86_DEBUG_FPU=y was set by auto regression framework
2. disable CONFIG_X86_DEBUG_FPU will bypass above WARNING and NULL
pointer issue
it may not make sense for general kernel regression check to enable
CONFIG_X86_DEBUG_FPU=y, will revise auto regression framework to keep
CONFIG_X86_DEBUG_FPU disabled to bypass it.
in the meanwhile, please let me know if this issue is still valuable to
look into.