MIPS: Support for hybrid FPRs

Hybrid FPRs is a scheme where scalar FP registers are 64b wide, but
accesses to odd indexed single registers use bits 63:32 of the
preceeding even indexed 64b register. In this mode all FP code
except that built for the plain FP64 ABI can execute correctly. Most
notably a combination of FP64A & FP32 code can execute correctly,
allowing for existing FP32 binaries to be linked with new FP64A binaries
that can make use of 64 bit FP & MSA.

Hybrid FPRs are implemented by setting both the FR & FRE bits, trapping
& emulating single precision FP instructions (via Reserved Instruction
exceptions) whilst allowing others to execute natively. It therefore has
a penalty in terms of execution speed, and should only be used when no
fully native mode can be. As more binaries are recompiled to use either
the FPXX or FP64(A) ABIs, the need for hybrid FPRs should diminish.
However in the short to mid term it allows for a gradual transition
towards that world, rather than a complete ABI break which is not
feasible for some users & not desirable for many.

A task will be executed using the hybrid FPR scheme when its
TIF_HYBRID_FPREGS flag is set & TIF_32BIT_FPREGS is clear. A further
patch will set the flags as necessary, this patch simply adds the
infrastructure necessary for the hybrid FPR mode to work.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/7683/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 47 insertions, 0 deletions

diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index 22b19c275044..165c275163b8 100644
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -724,6 +724,50 @@ int process_fpemu_return(int sig, void __user *fault_addr)
         }
 }
 
+static int simulate_fp(struct pt_regs *regs, unsigned int opcode,
+                       unsigned long old_epc, unsigned long old_ra)
+{
+        union mips_instruction inst = { .word = opcode };
+        void __user *fault_addr = NULL;
+        int sig;
+
+        /* If it's obviously not an FP instruction, skip it */
+        switch (inst.i_format.opcode) {
+        case cop1_op:
+        case cop1x_op:
+        case lwc1_op:
+        case ldc1_op:
+        case swc1_op:
+        case sdc1_op:
+                break;
+
+        default:
+                return -1;
+        }
+
+        /*
+         * do_ri skipped over the instruction via compute_return_epc, undo
+         * that for the FPU emulator.
+         */
+        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);
+
+        /* If something went wrong, signal */
+        process_fpemu_return(sig, fault_addr);
+
+        /* Restore the hardware register state */
+        own_fpu(1);
+
+        return 0;
+}
+
 /*
  * XXX Delayed fp exceptions when doing a lazy ctx switch XXX
  */
@@ -1016,6 +1060,9 @@ asmlinkage void do_ri(struct pt_regs *regs)
 
                 if (status < 0)
                         status = simulate_sync(regs, opcode);
+
+                if (status < 0)
+                        status = simulate_fp(regs, opcode, old_epc, old31);
         }
 
         if (status < 0)