MIPS: Basic MSA context switching support

This patch adds support for context switching the MSA vector registers.
These 128 bit vector registers are aliased with the FP registers - an
FP register accesses the least significant bits of the vector register
with which it is aliased (ie. the register with the same index). Due to
both this & the requirement that the scalar FPU must be 64-bit (FR=1) if
enabled at the same time as MSA the kernel will enable MSA & scalar FP
at the same time for tasks which use MSA. If we restore the MSA vector
context then we might as well enable the scalar FPU since the reason it
was left disabled was to allow for lazy FP context restoring - but we
just restored the FP context as it's a subset of the vector context. If
we restore the FP context and have previously used MSA then we have to
restore the whole vector context anyway (see comment in
enable_restore_fp_context for details) so similarly we might as well
enable MSA.

Thus if a task does not use MSA then it will continue to behave as
without this patch - the scalar FP context will be saved & restored as
usual. But if a task executes an MSA instruction then it will save &
restore the vector context forever more.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6431/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 8 insertions, 1 deletions

diff --git a/arch/mips/include/asm/processor.h b/arch/mips/include/asm/processor.h
index 50cf4c343118..ad70cba8daff 100644
--- a/arch/mips/include/asm/processor.h
+++ b/arch/mips/include/asm/processor.h
@@ -96,7 +96,12 @@ extern unsigned int vced_count, vcei_count;
 
 
 #define NUM_FPU_REGS    32
-#define FPU_REG_WIDTH   64
+
+#ifdef CONFIG_CPU_HAS_MSA
+# define FPU_REG_WIDTH  128
+#else
+# define FPU_REG_WIDTH  64
+#endif
 
 union fpureg {
         __u32   val32[FPU_REG_WIDTH / 32];
@@ -133,6 +138,7 @@ BUILD_FPR_ACCESS(64)
 struct mips_fpu_struct {
         union fpureg    fpr[NUM_FPU_REGS];
         unsigned int    fcr31;
+        unsigned int    msacsr;
 };
 
 #define NUM_DSP_REGS   6
@@ -310,6 +316,7 @@ struct thread_struct {
         .fpu                    = {                             \
                 .fpr            = {{{0,},},},                   \
                 .fcr31          = 0,                            \
+                .msacsr         = 0,                            \
         },                                                      \
         /*                                                      \
          * FPU affinity state (null if not FPAFF)               \