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MIPS: Simplify FP context initialization

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MIPS has up until now had 3 different ways for a task's floating point
context to be initialized:

  - If the task's first use of FP involves it gaining ownership of an
    FPU then _init_fpu() is used to initialize the FPU's registers such
    that they all contain ~0, and the FPU registers will be stored to
    struct thread_info later (eg. when context switching).

  - If the task first uses FP on a CPU without an associated FPU then
    fpu_emulator_init_fpu() initializes the task's floating point
    register state in struct thread_info such that all floating point
    register contain the bit pattern 0x7ff800007ff80000, different to
    the _init_fpu() behaviour.

  - If a task's floating point context is first accessed via ptrace then
    init_fp_ctx() initializes the floating point register state in
    struct thread_info to ~0, giving equivalent state to _init_fpu().

The _init_fpu() path has 2 separate implementations - one for r2k/r3k
style systems & one for r4k style systems. The _init_fpu() path also
requires that we be careful to clear & restore the value of the
Config5.FRE bit on modern systems in order to avoid inadvertently
triggering floating point exceptions.

None of this code is in a performance critical hot path - it runs only
the first time a task uses floating point. As such it doesn't seem to
warrant the complications of maintaining the _init_fpu() path.

Remove _init_fpu() & fpu_emulator_init_fpu(), instead using
init_fp_ctx() consistently to initialize floating point register state
in struct thread_info. Upon a task's first use of floating point this
will typically mean that we initialize state in memory & then load it
into FPU registers using _restore_fp() just as we would on a context
switch. For other paths such as __compute_return_epc_for_insn() or
mipsr2_decoder() this results in a significant simplification of the
work to be done.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21002/
Cc: linux-mips@linux-mips.org
This commit is contained in:
Paul Burton 2018-11-07 23:13:59 +00:00
parent b1013f1e95
commit cc97ab235f
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GPG key ID: 3EA79FACB57500DD
9 changed files with 38 additions and 292 deletions
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11
arch/mips/include/asm/fpu_emulator.h
View file

@ -188,17 +188,6 @@ int isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
int mm_isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
unsigned long *contpc);
#define SIGNALLING_NAN 0x7ff800007ff80000LL
static inline void fpu_emulator_init_fpu(void)
{
struct task_struct *t = current;
int i;
for (i = 0; i < 32; i++)
set_fpr64(&t->thread.fpu.fpr[i], 0, SIGNALLING_NAN);
}
/*
* Mask the FCSR Cause bits according to the Enable bits, observing
* that Unimplemented is always enabled.