Re: [PATCH v14 16/39] arm64/sme: Implement traps and syscall handling for SME

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On 2022/4/19 19:22, Mark Brown wrote:

diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index c938603b3ba0..92c69e5ac269 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -158,11 +158,36 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 	syscall_trace_exit(regs);
 }
-static inline void sve_user_discard(void)
+/*
+ * As per the ABI exit SME streaming mode and clear the SVE state not
+ * shared with FPSIMD on syscall entry.
+ */
+static inline void fp_user_discard(void)
 {
+	/*
+	 * If SME is active then exit streaming mode.  If ZA is active
+	 * then flush the SVE registers but leave userspace access to
+	 * both SVE and SME enabled, otherwise disable SME for the
+	 * task and fall through to disabling SVE too.  This means

It looks a bit confusing to me that in the current implementation, if
ZA is *not* active, we don't actually go to disable SME for the task
(which IMHO can be disabled as user-space is not actively using it now).

+	 * that after a syscall we never have any streaming mode
+	 * register state to track, if this changes the KVM code will
+	 * need updating.
+	 */
+	if (system_supports_sme() && test_thread_flag(TIF_SME)) {
+		u64 svcr = read_sysreg_s(SYS_SVCR_EL0);
+
+		if (svcr & SYS_SVCR_EL0_SM_MASK)
+			sme_smstop_sm();

As per the SME syscall ABI

| On syscall PSTATE.SM will be cleared and the SVE registers will be
| handled as per the standard SVE ABI.

and the SVE syscall ABI

| On syscall, V0..V31 are preserved (as without SVE).  Thus, bits
| [127:0] of Z0..Z31 are preserved.  All other bits of Z0..Z31, and all
| of P0..P15 and FFR become zero on return from a syscall.

Can we infer from the documentation that V0-V31 should be preserved on
return from a syscall? But with sme_smstop_sm(), all implemented bits of
Z0-Z31 are set to zero by hardware. Is this intentional?

Please fix me up if I've mis-read things here.

+	}
+
 	if (!system_supports_sve())
 		return;
+ /*
+	 * If SME is not active then disable SVE, the registers will
+	 * be cleared when userspace next attempts to access them and
+	 * we do not need to track the SVE register state until then.
+	 */
 	clear_thread_flag(TIF_SVE);
/*

Thanks,
Zenghui



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