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, 95 insertions, 6 deletions

diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index b0c7f80e05e5..4f27cbfad1e5 100644
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -47,6 +47,7 @@
 #include <asm/mipsregs.h>
 #include <asm/mipsmtregs.h>
 #include <asm/module.h>
+#include <asm/msa.h>
 #include <asm/pgtable.h>
 #include <asm/ptrace.h>
 #include <asm/sections.h>
@@ -79,6 +80,7 @@ extern asmlinkage void handle_ov(void);
 extern asmlinkage void handle_tr(void);
 extern asmlinkage void handle_fpe(void);
 extern asmlinkage void handle_ftlb(void);
+extern asmlinkage void handle_msa(void);
 extern asmlinkage void handle_mdmx(void);
 extern asmlinkage void handle_watch(void);
 extern asmlinkage void handle_mt(void);
@@ -1074,6 +1076,76 @@ static int default_cu2_call(struct notifier_block *nfb, unsigned long action,
 	return NOTIFY_OK;
 }
 
+static int enable_restore_fp_context(int msa)
+{
+	int err, was_fpu_owner;
+
+	if (!used_math()) {
+		/* First time FP context user. */
+		err = init_fpu();
+		if (msa && !err)
+			enable_msa();
+		if (!err)
+			set_used_math();
+		return err;
+	}
+
+	/*
+	 * This task has formerly used the FP context.
+	 *
+	 * If this thread has no live MSA vector context then we can simply
+	 * restore the scalar FP context. If it has live MSA vector context
+	 * (that is, it has or may have used MSA since last performing a
+	 * function call) then we'll need to restore the vector context. This
+	 * applies even if we're currently only executing a scalar FP
+	 * instruction. This is because if we were to later execute an MSA
+	 * instruction then we'd either have to:
+	 *
+	 *  - Restore the vector context & clobber any registers modified by
+	 *    scalar FP instructions between now & then.
+	 *
+	 * or
+	 *
+	 *  - Not restore the vector context & lose the most significant bits
+	 *    of all vector registers.
+	 *
+	 * Neither of those options is acceptable. We cannot restore the least
+	 * significant bits of the registers now & only restore the most
+	 * significant bits later because the most significant bits of any
+	 * vector registers whose aliased FP register is modified now will have
+	 * been zeroed. We'd have no way to know that when restoring the vector
+	 * context & thus may load an outdated value for the most significant
+	 * bits of a vector register.
+	 */
+	if (!msa && !thread_msa_context_live())
+		return own_fpu(1);
+
+	/*
+	 * This task is using or has previously used MSA. Thus we require
+	 * that Status.FR == 1.
+	 */
+	was_fpu_owner = is_fpu_owner();
+	err = own_fpu(0);
+	if (err)
+		return err;
+
+	enable_msa();
+	write_msa_csr(current->thread.fpu.msacsr);
+	set_thread_flag(TIF_USEDMSA);
+
+	/*
+	 * If this is the first time that the task is using MSA and it has
+	 * previously used scalar FP in this time slice then we already nave
+	 * FP context which we shouldn't clobber.
+	 */
+	if (!test_and_set_thread_flag(TIF_MSA_CTX_LIVE) && was_fpu_owner)
+		return 0;
+
+	/* We need to restore the vector context. */
+	restore_msa(current);
+	return 0;
+}
+
 asmlinkage void do_cpu(struct pt_regs *regs)
 {
 	enum ctx_state prev_state;
@@ -1153,12 +1225,7 @@ asmlinkage void do_cpu(struct pt_regs *regs)
 		/* Fall through.  */
 
 	case 1:
-		if (used_math())	/* Using the FPU again.	 */
-			err = own_fpu(1);
-		else {			/* First time FPU user.	 */
-			err = init_fpu();
-			set_used_math();
-		}
+		err = enable_restore_fp_context(0);
 
 		if (!raw_cpu_has_fpu || err) {
 			int sig;
@@ -1183,6 +1250,27 @@ out:
 	exception_exit(prev_state);
 }
 
+asmlinkage void do_msa(struct pt_regs *regs)
+{
+	enum ctx_state prev_state;
+	int err;
+
+	prev_state = exception_enter();
+
+	if (!cpu_has_msa || test_thread_flag(TIF_32BIT_FPREGS)) {
+		force_sig(SIGILL, current);
+		goto out;
+	}
+
+	die_if_kernel("do_msa invoked from kernel context!", regs);
+
+	err = enable_restore_fp_context(1);
+	if (err)
+		force_sig(SIGILL, current);
+out:
+	exception_exit(prev_state);
+}
+
 asmlinkage void do_mdmx(struct pt_regs *regs)
 {
 	enum ctx_state prev_state;
@@ -2041,6 +2129,7 @@ void __init trap_init(void)
 		set_except_vector(15, handle_fpe);
 
 	set_except_vector(16, handle_ftlb);
+	set_except_vector(21, handle_msa);
 	set_except_vector(22, handle_mdmx);
 
 	if (cpu_has_mcheck)