MIPS: Ensure emulated FP sets PF_USED_MATH

Emulated floating point instructions don't ensure that the PF_USED_MATH
flag is set for the task. This results in a couple of inconsistencies:

  - ptrace will return the default initial state of FP registers rather
    than the values actually stored in struct thread_struct, hiding
    state that has been updated by emulated floating point instructions.

  - If a task migrates to a CPU with an FPU after having emulated
    floating point instructions then its floating point register state
    will be reset to the default ~0 bit pattern, losing state from the
    emulated instructions.

Fix this by calling init_fp_ctx() from fpu_emulator_cop1Handler() to
consistently initialize FP state if it was previously uninitialized,
setting the PF_USED_MATH flag in the process.

All callers of fpu_emulator_cop1Handler() either call lose_fpu(1) before
it in order to save any live FPU registers to struct thread_struct, or
in the case of do_cpu() already know that the task does not own an FPU
so lose_fpu(1) would be a no-op. Since we know that saving FP context
will be unnecessary in the case where FP context was just initialized we
move this call into fpu_emulator_cop1Handler() too, providing
consistency & avoiding needless duplication.

Calls to own_fpu(1) are common after return from
fpu_emulator_cop1Handler() too, but this would not be a no-op in the
do_cpu() case so these are left as-is. A potential future improvement
could be to have fpu_emulator_cop1Handler() restore FPU state
automatically only if it saved it, though this may not be optimal if
some callers are better off without their current calls to own_fpu(1).
One potential example of this could be mipsr2_decoder() which as-is
could end up saving & restoring FP context repeatedly & unnecessarily if
emulating multiple FP instructions.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21003/
Cc: linux-mips@linux-mips.org

diff --git a/arch/mips/kernel/mips-r2-to-r6-emul.c b/arch/mips/kernel/mips-r2-to-r6-emul.c
index 6092cdf7f8d9594af3fd1e77e408b5eb57a4076f..cb22a558431e21ce4fca50fc87a51311f2fdc27e 100644 (file)
--- a/arch/mips/kernel/mips-r2-to-r6-emul.c
+++ b/arch/mips/kernel/mips-r2-to-r6-emul.c
@@ -1175,9 +1175,6 @@ fpu_emul:
                regs->regs[31] = r31;
                regs->cp0_epc = epc;
 
-               if (!init_fp_ctx(current))
-                       lose_fpu(1);
-
                err = fpu_emulator_cop1Handler(regs, &current->thread.fpu, 0,
                                               &fault_addr);
 

diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index 650b9dd05b8ef829d1ea7e8a322862658651cad1..db52d30eacec51e9ebde7c3ba95569c3bedc3321 100644 (file)
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -794,9 +794,6 @@ static int simulate_fp(struct pt_regs *regs, unsigned int opcode,
        regs->cp0_epc = old_epc;
        regs->regs[31] = old_ra;
 
-       /* Save the FP context to struct thread_struct */
-       lose_fpu(1);
-
        /* Run the emulator */
        sig = fpu_emulator_cop1Handler(regs, &current->thread.fpu, 1,
                                       &fault_addr);
@@ -848,8 +845,6 @@ asmlinkage void do_fpe(struct pt_regs *regs, unsigned long fcr31)
                 * register operands before invoking the emulator, which seems
                 * a bit extreme for what should be an infrequent event.
                 */
-               /* Ensure 'resume' not overwrite saved fp context again. */
-               lose_fpu(1);
 
                /* Run the emulator */
                sig = fpu_emulator_cop1Handler(regs, &current->thread.fpu, 1,

diff --git a/arch/mips/kernel/unaligned.c b/arch/mips/kernel/unaligned.c
index ce446eed62d28eb969fd217624ccab9cc6ae068d..4c7de9f6373d7e4249e05903ed3d0f72fdd480ac 100644 (file)
--- a/arch/mips/kernel/unaligned.c
+++ b/arch/mips/kernel/unaligned.c
@@ -1220,7 +1220,6 @@ static void emulate_load_store_insn(struct pt_regs *regs,
                die_if_kernel("Unaligned FP access in kernel code", regs);
                BUG_ON(!used_math());
 
-               lose_fpu(1);    /* Save FPU state for the emulator. */
                res = fpu_emulator_cop1Handler(regs, &current->thread.fpu, 1,
                                               &fault_addr);
                own_fpu(1);     /* Restore FPU state. */
@@ -1733,7 +1732,6 @@ fpu_emul:
                BUG_ON(!used_math());
                BUG_ON(!is_fpu_owner());
 
-               lose_fpu(1);    /* save the FPU state for the emulator */
                res = fpu_emulator_cop1Handler(regs, &current->thread.fpu, 1,
                                               &fault_addr);
                own_fpu(1);     /* restore FPU state */

diff --git a/arch/mips/math-emu/cp1emu.c b/arch/mips/math-emu/cp1emu.c
index 62deb025970bdd5391f1288a554fa24d14070bf5..82e2993c1a2c49c3fcb31b803abb60642ade1f30 100644 (file)
--- a/arch/mips/math-emu/cp1emu.c
+++ b/arch/mips/math-emu/cp1emu.c
@@ -2831,6 +2831,13 @@ int fpu_emulator_cop1Handler(struct pt_regs *xcp, struct mips_fpu_struct *ctx,
        u16 *instr_ptr;
        int sig = 0;
 
+       /*
+        * Initialize context if it hasn't been used already, otherwise ensure
+        * it has been saved to struct thread_struct.
+        */
+       if (!init_fp_ctx(current))
+               lose_fpu(1);
+
        oldepc = xcp->cp0_epc;
        do {
                prevepc = xcp->cp0_epc;