[patch 01/21] x86/fpu: Provide struct fpstate

Thomas Gleixner
2021-10-13T14:55:34Z

New xfeatures will not longer be automatically stored in the regular XSAVE
buffer in thread_struct::fpu.

The kernel will provide the default sized buffer for storing the regular
features up to AVX512 in thread_struct::fpu and if a task requests to use
one of the new features then the register storage has to be extendend.

The state will be accessed via a pointer in thread_struct::fpu which
defaults to the builtin storage and can be switched when extended storage
is required.

To avoid conditionals all over the code, create a new container for the
register storage which will gain other information, e.g. size, feature
masks etc., later. For now it just contains the register storage, which
gives it exactly the same layout as the exiting fpu::state.

Stick fpu::state and the new fpu::__fpstate into an anonymous union and
initialize the pointer. Add build time checks to validate that both are
at the same place and have the same size.

This allows step by step conversion of all users.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/include/asm/fpu/types.h | 20 +++++++++++++++++++-
arch/x86/include/asm/processor.h | 4 ++--
arch/x86/kernel/fpu/core.c | 11 ++++++++++-
arch/x86/kernel/fpu/init.c | 9 +++++++--
arch/x86/kernel/fpu/internal.h | 1 +
5 files changed, 39 insertions(+), 6 deletions(-)

--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -309,6 +309,13 @@ union fpregs_state {
u8 __padding[PAGE_SIZE];
};

+struct fpstate {
+ /* @regs: The register state union for all supported formats */
+ union fpregs_state regs;
+
+ /* @regs is dynamically sized! Don't add anything after @regs! */
+} __attribute__ ((aligned (64)));
+
/*
* Highest level per task FPU state data structure that
* contains the FPU register state plus various FPU
@@ -337,6 +344,14 @@ struct fpu {
unsigned long avx512_timestamp;

/*
+ * @fpstate:
+ *
+ * Pointer to the active struct fpstate. Initialized to
+ * point at @__fpstate below.
+ */
+ struct fpstate *fpstate;
+
+ /*
* @state:
*
* In-memory copy of all FPU registers that we save/restore
@@ -345,7 +360,10 @@ struct fpu {
* copy. If the task context-switches away then they get
* saved here and represent the FPU state.
*/
- union fpregs_state state;
+ union {
+ struct fpstate __fpstate;
+ union fpregs_state state;
+ };
/*
* WARNING: 'state' is dynamically-sized. Do not put
* anything after it here.
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -537,11 +537,11 @@ struct thread_struct {
*/
};

-/* Whitelist the FPU state from the task_struct for hardened usercopy. */
+/* Whitelist the FPU register state from the task_struct for hardened usercopy. */
static inline void arch_thread_struct_whitelist(unsigned long *offset,
unsigned long *size)
{
- *offset = offsetof(struct thread_struct, fpu.state);
+ *offset = offsetof(struct thread_struct, fpu.__fpstate.regs);
*size = fpu_kernel_xstate_size;
}

--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -337,10 +337,17 @@ void fpstate_init_user(union fpregs_stat
fpstate_init_fstate(&state->fsave);
}

+void fpstate_reset(struct fpu *fpu)
+{
+ /* Set the fpstate pointer to the default fpstate */
+ fpu->fpstate = &fpu->__fpstate;
+}
+
#if IS_ENABLED(CONFIG_KVM)
void fpu_init_fpstate_user(struct fpu *fpu)
{
- fpstate_init_user(&fpu->state);
+ fpstate_reset(fpu);
+ fpstate_init_user(&fpu->fpstate->regs);
}
EXPORT_SYMBOL_GPL(fpu_init_fpstate_user);
#endif
@@ -354,6 +361,8 @@ int fpu_clone(struct task_struct *dst)
/* The new task's FPU state cannot be valid in the hardware. */
dst_fpu->last_cpu = -1;

+ fpstate_reset(dst_fpu);
+
if (!cpu_feature_enabled(X86_FEATURE_FPU))
return 0;

--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -165,7 +165,7 @@ static void __init fpu__init_task_struct
* Subtract off the static size of the register state.
* It potentially has a bunch of padding.
*/
- task_size -= sizeof(((struct task_struct *)0)->thread.fpu.state);
+ task_size -= sizeof(current->thread.fpu.__fpstate.regs);

/*
* Add back the dynamically-calculated register state
@@ -180,10 +180,14 @@ static void __init fpu__init_task_struct
* you hit a compile error here, check the structure to
* see if something got added to the end.
*/
- CHECK_MEMBER_AT_END_OF(struct fpu, state);
+ CHECK_MEMBER_AT_END_OF(struct fpu, __fpstate);
CHECK_MEMBER_AT_END_OF(struct thread_struct, fpu);
CHECK_MEMBER_AT_END_OF(struct task_struct, thread);

+ BUILD_BUG_ON(sizeof(struct fpstate) != sizeof(union fpregs_state));
+ BUILD_BUG_ON(offsetof(struct thread_struct, fpu.state) !=
+ offsetof(struct thread_struct, fpu.__fpstate));
+
arch_task_struct_size = task_size;
}

@@ -220,6 +224,7 @@ static void __init fpu__init_system_xsta
*/
void __init fpu__init_system(struct cpuinfo_x86 *c)
{
+ fpstate_reset(&current->thread.fpu);
fpu__init_system_early_generic(c);

/*
--- a/arch/x86/kernel/fpu/internal.h
+++ b/arch/x86/kernel/fpu/internal.h
@@ -26,5 +26,6 @@ extern void fpu__init_prepare_fx_sw_fram

/* Used in init.c */
extern void fpstate_init_user(union fpregs_state *state);
+extern void fpstate_reset(struct fpu *fpu);

#endif