[PATCH] powerpc/pseries: Fix xive=off command line

[Greg Kurz <groug@kaod.org>]

On POWER9, if the hypervisor supports XIVE exploitation mode, the guest OS
will unconditionally requests for the XIVE interrupt mode even if XIVE was
deactivated with the kernel command line xive=off. Later on, when the spapr
XIVE init code handles xive=off, it disables XIVE and tries to fall back on
the legacy mode XICS.

This discrepency causes a kernel panic because the hypervisor is configured
to provide the XIVE interrupt mode to the guest :

[    0.008837] kernel BUG at arch/powerpc/sysdev/xics/xics-common.c:135!
[    0.008877] Oops: Exception in kernel mode, sig: 5 [#1]
[    0.008908] LE SMP NR_CPUS=1024 NUMA pSeries
[    0.008939] Modules linked in:
[    0.008964] CPU: 0 PID: 1 Comm: swapper/0 Tainted: G        W         5.0.13-200.fc29.ppc64le #1
[    0.009018] NIP:  c000000001029ab8 LR: c000000001029aac CTR: c0000000018e0000
[    0.009065] REGS: c0000007f96d7900 TRAP: 0700   Tainted: G        W          (5.0.13-200.fc29.ppc64le)
[    0.009119] MSR:  8000000002029033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 28000222  XER: 20040000
[    0.009168] CFAR: c0000000001b1e28 IRQMASK: 0
[    0.009168] GPR00: c000000001029aac c0000007f96d7b90 c0000000015e8600 0000000000000000
[    0.009168] GPR04: 0000000000000001 0000000000000000 0000000000000061 646f6d61696e0d0a
[    0.009168] GPR08: 00000007fd8f0000 0000000000000001 c0000000014c44c0 c0000007f96d76cf
[    0.009168] GPR12: 0000000000000000 c0000000018e0000 0000000000000001 0000000000000000
[    0.009168] GPR16: 0000000000000000 0000000000000001 c0000007f96d7c08 c0000000016903d0
[    0.009168] GPR20: c0000007fffe04e8 ffffffffffffffea c000000001620164 c00000000161fe58
[    0.009168] GPR24: c000000000ea6c88 c0000000011151a8 00000000006000c0 c0000007f96d7c34
[    0.009168] GPR28: 0000000000000000 c0000000014b286c c000000001115180 c00000000161dc70
[    0.009558] NIP [c000000001029ab8] xics_smp_probe+0x38/0x98
[    0.009590] LR [c000000001029aac] xics_smp_probe+0x2c/0x98
[    0.009622] Call Trace:
[    0.009639] [c0000007f96d7b90] [c000000001029aac] xics_smp_probe+0x2c/0x98 (unreliable)
[    0.009687] [c0000007f96d7bb0] [c000000001033404] pSeries_smp_probe+0x40/0xa0
[    0.009734] [c0000007f96d7bd0] [c0000000010212a4] smp_prepare_cpus+0x62c/0x6ec
[    0.009782] [c0000007f96d7cf0] [c0000000010141b8] kernel_init_freeable+0x148/0x448
[    0.009829] [c0000007f96d7db0] [c000000000010ba4] kernel_init+0x2c/0x148
[    0.009870] [c0000007f96d7e20] [c00000000000bdd4] ret_from_kernel_thread+0x5c/0x68
[    0.009916] Instruction dump:
[    0.009940] 7c0802a6 60000000 7c0802a6 38800002 f8010010 f821ffe1 3c62001c e863b9a0
[    0.009988] 4b1882d1 60000000 7c690034 5529d97e <0b090000> 3d22001c e929b998 3ce2ff8f

Look for xive=off during prom_init and don't ask for XIVE in this case. One
exception though: if the host only supports XIVE, we still want to boot so
we ignore xive=off.

Similarly, have the spapr XIVE init code to looking at the interrupt mode
negociated during CAS, and ignore xive=off if the hypervisor only supports
XIVE.

Fixes: eac1e731b59e ("powerpc/xive: guest exploitation of the XIVE interrupt controller")
Cc: stable@vger.kernel.org # v4.20
Reported-by: Pavithra R. Prakash <pavrampu@in.ibm.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
---
eac1e731b59e is a v4.16 commit actually but this patch only applies
cleanly to v4.20 and newer. If needed I can send a backport for
older versions.
---
 arch/powerpc/kernel/prom_init.c  |   16 +++++++++++-
 arch/powerpc/sysdev/xive/spapr.c |   52 +++++++++++++++++++++++++++++++++++++-
 2 files changed, 66 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/prom_init.c b/arch/powerpc/kernel/prom_init.c
index 523bb99d7676..c8f7eb845927 100644
--- a/arch/powerpc/kernel/prom_init.c
+++ b/arch/powerpc/kernel/prom_init.c
@@ -172,6 +172,7 @@ static unsigned long __prombss prom_tce_alloc_end;
 
 #ifdef CONFIG_PPC_PSERIES
 static bool __prombss prom_radix_disable;
+static bool __prombss prom_xive_disable;
 #endif
 
 struct platform_support {
@@ -808,6 +809,12 @@ static void __init early_cmdline_parse(void)
 	}
 	if (prom_radix_disable)
 		prom_debug("Radix disabled from cmdline\n");
+
+	opt = prom_strstr(prom_cmd_line, "xive=off");
+	if (opt) {
+		prom_xive_disable = true;
+		prom_debug("XIVE disabled from cmdline\n");
+	}
 #endif /* CONFIG_PPC_PSERIES */
 }
 
@@ -1216,10 +1223,17 @@ static void __init prom_parse_xive_model(u8 val,
 	switch (val) {
 	case OV5_FEAT(OV5_XIVE_EITHER): /* Either Available */
 		prom_debug("XIVE - either mode supported\n");
-		support->xive = true;
+		support->xive = !prom_xive_disable;
 		break;
 	case OV5_FEAT(OV5_XIVE_EXPLOIT): /* Only Exploitation mode */
 		prom_debug("XIVE - exploitation mode supported\n");
+		if (prom_xive_disable) {
+			/*
+			 * If we __have__ to do XIVE, we're better off ignoring
+			 * the command line rather than not booting.
+			 */
+			prom_printf("WARNING: Ignoring cmdline option xive=off\n");
+		}
 		support->xive = true;
 		break;
 	case OV5_FEAT(OV5_XIVE_LEGACY): /* Only Legacy mode */
diff --git a/arch/powerpc/sysdev/xive/spapr.c b/arch/powerpc/sysdev/xive/spapr.c
index 575db3b06a6b..2e2d1b8f810f 100644
--- a/arch/powerpc/sysdev/xive/spapr.c
+++ b/arch/powerpc/sysdev/xive/spapr.c
@@ -20,6 +20,7 @@
 #include <linux/cpumask.h>
 #include <linux/mm.h>
 #include <linux/delay.h>
+#include <linux/libfdt.h>
 
 #include <asm/prom.h>
 #include <asm/io.h>
@@ -663,6 +664,55 @@ static bool xive_get_max_prio(u8 *max_prio)
 	return true;
 }
 
+static const u8 *get_vec5_feature(unsigned int index)
+{
+	unsigned long root, chosen;
+	int size;
+	const u8 *vec5;
+
+	root = of_get_flat_dt_root();
+	chosen = of_get_flat_dt_subnode_by_name(root, "chosen");
+	if (chosen == -FDT_ERR_NOTFOUND)
+		return NULL;
+
+	vec5 = of_get_flat_dt_prop(chosen, "ibm,architecture-vec-5", &size);
+	if (!vec5)
+		return NULL;
+
+	if (size <= index)
+		return NULL;
+
+	return vec5 + index;
+}
+
+static bool xive_spapr_disabled(void)
+{
+	const u8 *vec5_xive;
+
+	vec5_xive = get_vec5_feature(OV5_INDX(OV5_XIVE_SUPPORT));
+	if (vec5_xive) {
+		u8 val;
+
+		val = *vec5_xive & OV5_FEAT(OV5_XIVE_SUPPORT);
+		switch (val) {
+		case OV5_FEAT(OV5_XIVE_EITHER):
+		case OV5_FEAT(OV5_XIVE_LEGACY):
+			break;
+		case OV5_FEAT(OV5_XIVE_EXPLOIT):
+			/* Hypervisor only supports XIVE */
+			if (xive_cmdline_disabled)
+				pr_warn("WARNING: Ignoring cmdline option xive=off\n");
+			return false;
+		default:
+			pr_warn("%s: Unknown xive support option: 0x%x\n",
+				__func__, val);
+			break;
+		}
+	}
+
+	return xive_cmdline_disabled;
+}
+
 bool __init xive_spapr_init(void)
 {
 	struct device_node *np;
@@ -675,7 +725,7 @@ bool __init xive_spapr_init(void)
 	const __be32 *reg;
 	int i;
 
-	if (xive_cmdline_disabled)
+	if (xive_spapr_disabled())
 		return false;
 
 	pr_devel("%s()\n", __func__);