Commit-ID: af7f8721f1f1252473b154c38dd7583abfe3206b
Gitweb: http://git.kernel.org/tip/af7f8721f1f1252473b154c38dd7583abfe3206b
Author: Ingo Molnar <mingo@xxxxxxxxxx>
AuthorDate: Thu, 23 Apr 2015 14:06:05 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 19 May 2015 15:47:25 +0200
x86/fpu: Document fpu__unlazy_stopped()
Explain its usage and also document a TODO item.
Reviewed-by: Borislav Petkov <bp@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/kernel/fpu/core.c | 25 +++++++++++++++++++++----
1 file changed, 21 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 90f624d..7798131 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -284,10 +284,27 @@ int fpstate_alloc_init(struct task_struct *curr)
EXPORT_SYMBOL_GPL(fpstate_alloc_init);
/*
- * The _current_ task is using the FPU for the first time
- * so initialize it and set the mxcsr to its default
- * value at reset if we support XMM instructions and then
- * remember the current task has used the FPU.
+ * This function is called before we modify a stopped child's
+ * FPU state context.
+ *
+ * If the child has not used the FPU before then initialize its
+ * FPU context.
+ *
+ * If the child has used the FPU before then unlazy it.
+ *
+ * [ After this function call, after the context is modified and
+ * the child task is 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
+ * state pending on its former CPU could be restored, losing
+ * the modifications. ]
+ *
+ * This function is also called before we read a stopped child's
+ * FPU state - to make sure it's modified.
+ *
+ * TODO: A future optimization would be to skip the unlazying in
+ * the read-only case, it's not strictly necessary for
+ * read-only access to the context.
*/
static int fpu__unlazy_stopped(struct task_struct *child)
{
--
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