linux-kernel - [PATCH 30/33] x86/fpu: Fix stale comments about lazy FPU logic

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Date:   Sat, 23 Sep 2017 15:00:13 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Eric Biggers <ebiggers3@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: [PATCH 30/33] x86/fpu: Fix stale comments about lazy FPU logic

We don't do any lazy restore anymore, what we have are two pieces of optimization:

 - no-FPU tasks that don't save/restore the FPU context (kernel threads are such)

 - cached FPU registers maintained via the fpu->last_cpu field. This means that
   if an FPU task context switches to a non-FPU task then we can maintain the
   FPU registers as an in-FPU copies (cache), and skip the restoration of them
   once we switch back to the original FPU-using task.

Update all the comments that still referred to old 'lazy' and 'unlazy' concepts.

Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Eric Biggers <ebiggers3@...il.com>
Cc: Fenghua Yu <fenghua.yu@...el.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/fpu/core.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index d770f9a6d4e1..4acfc0ebc160 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -205,9 +205,6 @@ int fpu__copy(struct fpu *dst_fpu, struct fpu *src_fpu)
 	/*
 	 * Save current FPU registers directly into the child
 	 * FPU context, without any memory-to-memory copying.
-	 * In lazy mode, if the FPU context isn't loaded into
-	 * fpregs, CR0.TS will be set and do_device_not_available
-	 * will load the FPU context.
 	 *
 	 * We have to do all this with preemption disabled,
 	 * mostly because of the FNSAVE case, because in that
@@ -274,13 +271,13 @@ void fpu__activate_fpstate_read(struct fpu *fpu)
 /*
  * This function must be called before we write a task's fpstate.
  *
- * If the task has used the FPU before then unlazy it.
+ * If the task has used the FPU before then invalidate any cached FPU registers.
  * If the task has not used the FPU before then initialize its fpstate.
  *
  * After this function call, after registers in the fpstate are
  * modified and the child task has woken up, the child task will
  * restore the modified FPU state from the modified context. If we
- * didn't clear its lazy status here then the lazy in-registers
+ * didn't clear its cached status here then the cached in-registers
  * state pending on its former CPU could be restored, corrupting
  * the modifications.
  */
@@ -293,7 +290,7 @@ void fpu__activate_fpstate_write(struct fpu *fpu)
 	WARN_ON_FPU(fpu == &current->thread.fpu);
 
 	if (fpu->initialized) {
-		/* Invalidate any lazy state: */
+		/* Invalidate any cached state: */
 		__fpu_invalidate_fpregs_state(fpu);
 	} else {
 		fpstate_init(&fpu->state);
-- 
2.11.0