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Subject[PATCH 197/208] x86/fpu: Change fpu->fpregs_active from 'int' to 'char', add lazy switching comments
Date
Improve the memory layout of 'struct fpu':

- change ->fpregs_active from 'int' to 'char' - it's just a single flag
and modern x86 CPUs can do efficient byte accesses.

- pack related fields closer to each other: often 'fpu->state' will not be
touched, while the other fields will - so pack them into a group.

Also add comments to each field, describing their purpose, and add
some background information about lazy restores.

Also fix an obsolete, lazy switching related comment in fpu_copy()'s description.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/include/asm/fpu/types.h | 82 ++++++++++++++++++++++++++++++++++++++++++++++++++++--------
arch/x86/kernel/fpu/core.c | 6 ++---
arch/x86/kernel/fpu/xstate.c | 9 ++++---
3 files changed, 79 insertions(+), 18 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index fe2ce3276a38..261cfb76065f 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -159,8 +159,44 @@ union fpregs_state {

struct fpu {
/*
+ * @state:
+ *
+ * In-memory copy of all FPU registers that we save/restore
+ * over context switches. If the task is using the FPU then
+ * the registers in the FPU are more recent than this state
+ * copy. If the task context-switches away then they get
+ * saved here and represent the FPU state.
+ *
+ * After context switches there may be a (short) time period
+ * during which the in-FPU hardware registers are unchanged
+ * and still perfectly match this state, if the tasks
+ * scheduled afterwards are not using the FPU.
+ *
+ * This is the 'lazy restore' window of optimization, which
+ * we track though 'fpu_fpregs_owner_ctx' and 'fpu->last_cpu'.
+ *
+ * We detect whether a subsequent task uses the FPU via setting
+ * CR0::TS to 1, which causes any FPU use to raise a #NM fault.
+ *
+ * During this window, if the task gets scheduled again, we
+ * might be able to skip having to do a restore from this
+ * memory buffer to the hardware registers - at the cost of
+ * incurring the overhead of #NM fault traps.
+ *
+ * Note that on modern CPUs that support the XSAVEOPT (or other
+ * optimized XSAVE instructions), we don't use #NM traps anymore,
+ * as the hardware can track whether FPU registers need saving
+ * or not. On such CPUs we activate the non-lazy ('eagerfpu')
+ * logic, which unconditionally saves/restores all FPU state
+ * across context switches. (if FPU state exists.)
+ */
+ union fpregs_state state;
+
+ /*
+ * @last_cpu:
+ *
* Records the last CPU on which this context was loaded into
- * FPU registers. (In the lazy-switching case we might be
+ * FPU registers. (In the lazy-restore case we might be
* able to reuse FPU registers across multiple context switches
* this way, if no intermediate task used the FPU.)
*
@@ -170,23 +206,49 @@ struct fpu {
*/
unsigned int last_cpu;

- unsigned int fpregs_active;
- union fpregs_state state;
/*
+ * @fpstate_active:
+ *
+ * This flag indicates whether this context is active: if the task
+ * is not running then we can restore from this context, if the task
+ * is running then we should save into this context.
+ */
+ unsigned char fpstate_active;
+
+ /*
+ * @fpregs_active:
+ *
+ * This flag determines whether a given context is actively
+ * loaded into the FPU's registers and that those registers
+ * represent the task's current FPU state.
+ *
+ * Note the interaction with fpstate_active:
+ *
+ * # task does not use the FPU:
+ * fpstate_active == 0
+ *
+ * # task uses the FPU and regs are active:
+ * fpstate_active == 1 && fpregs_active == 1
+ *
+ * # the regs are inactive but still match fpstate:
+ * fpstate_active == 1 && fpregs_active == 0 && fpregs_owner == fpu
+ *
+ * The third state is what we use for the lazy restore optimization
+ * on lazy-switching CPUs.
+ */
+ unsigned char fpregs_active;
+
+ /*
+ * @counter:
+ *
* This counter contains the number of consecutive context switches
* during which the FPU stays used. If this is over a threshold, the
- * lazy fpu saving logic becomes unlazy, to save the trap overhead.
+ * lazy FPU restore logic becomes eager, to save the trap overhead.
* This is an unsigned char so that after 256 iterations the counter
* wraps and the context switch behavior turns lazy again; this is to
* deal with bursty apps that only use the FPU for a short time:
*/
unsigned char counter;
- /*
- * This flag indicates whether this context is fpstate_active: if the task is
- * not running then we can restore from this context, if the task
- * is running then we should save into this context.
- */
- unsigned char fpstate_active;
};

#endif /* _ASM_X86_FPU_H */
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 0acdfc5f8d19..63496c49a590 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -227,10 +227,8 @@ EXPORT_SYMBOL_GPL(fpstate_init);
/*
* Copy the current task's FPU state to a new task's FPU context.
*
- * In the 'eager' case we just save to the destination context.
- *
- * In the 'lazy' case we save to the source context, mark the FPU lazy
- * via stts() and copy the source context into the destination context.
+ * In both the 'eager' and the 'lazy' case we save hardware registers
+ * directly to the destination buffer.
*/
static void fpu_copy(struct fpu *dst_fpu, struct fpu *src_fpu)
{
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 733a8aec7bd7..cd7f1a6bd933 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -76,10 +76,11 @@ int cpu_has_xfeatures(u64 xfeatures_needed, const char **feature_name)
EXPORT_SYMBOL_GPL(cpu_has_xfeatures);

/*
- * When executing XSAVEOPT (optimized XSAVE), if a processor implementation
- * detects that an FPU state component is still (or is again) in its
- * initialized state, it may clear the corresponding bit in the header.xfeatures
- * field, and can skip the writeout of registers to the corresponding memory layout.
+ * When executing XSAVEOPT (or other optimized XSAVE instructions), if
+ * a processor implementation detects that an FPU state component is still
+ * (or is again) in its initialized state, it may clear the corresponding
+ * bit in the header.xfeatures field, and can skip the writeout of registers
+ * to the corresponding memory layout.
*
* This means that when the bit is zero, the state component might still contain
* some previous - non-initialized register state.
--
2.1.0


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