powerpc: Set crashkernel offset to mid of RMA region

On large config LPARs (having 192 and more cores), Linux fails to boot
due to insufficient memory in the first memblock. It is due to the
memory reservation for the crash kernel which starts at 128MB offset of
the first memblock. This memory reservation for the crash kernel doesn't
leave enough space in the first memblock to accommodate other essential
system resources.

The crash kernel start address was set to 128MB offset by default to
ensure that the crash kernel get some memory below the RMA region which
is used to be of size 256MB. But given that the RMA region size can be
512MB or more, setting the crash kernel offset to mid of RMA size will
leave enough space for the kernel to allocate memory for other system
resources.

Since the above crash kernel offset change is only applicable to the LPAR
platform, the LPAR feature detection is pushed before the crash kernel
reservation. The rest of LPAR specific initialization will still
be done during pseries_probe_fw_features as usual.

This patch is dependent on changes to paca allocation for boot CPU. It
expect boot CPU to discover 1T segment support which is introduced by
the patch posted here:
https://lists.ozlabs.org/pipermail/linuxppc-dev/2022-January/239175.htmlReported-by: Abdul haleem <abdhalee@linux.vnet.ibm.com>
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220204085601.107257-1-sourabhjain@linux.ibm.com

arch/powerpc/kernel/rtas.c

@@ -1313,6 +1313,12 @@ int __init early_init_dt_scan_rtas(unsigned long node,
 	entryp = of_get_flat_dt_prop(node, "linux,rtas-entry", NULL);
 	sizep  = of_get_flat_dt_prop(node, "rtas-size", NULL);
 
+#ifdef CONFIG_PPC64
+	/* need this feature to decide the crashkernel offset */
+	if (of_get_flat_dt_prop(node, "ibm,hypertas-functions", NULL))
+		powerpc_firmware_features |= FW_FEATURE_LPAR;
+#endif
+
 	if (basep && entryp && sizep) {
 		rtas.base = *basep;
 		rtas.entry = *entryp;

arch/powerpc/kexec/core.c

@@ -134,11 +134,18 @@ void __init reserve_crashkernel(void)
 	if (!crashk_res.start) {
 #ifdef CONFIG_PPC64
 		/*
-		 * On 64bit we split the RMO in half but cap it at half of
-		 * a small SLB (128MB) since the crash kernel needs to place
-		 * itself and some stacks to be in the first segment.
+		 * On the LPAR platform place the crash kernel to mid of
+		 * RMA size (512MB or more) to ensure the crash kernel
+		 * gets enough space to place itself and some stack to be
+		 * in the first segment. At the same time normal kernel
+		 * also get enough space to allocate memory for essential
+		 * system resource in the first segment. Keep the crash
+		 * kernel starts at 128MB offset on other platforms.
 		 */
-		crashk_res.start = min(0x8000000ULL, (ppc64_rma_size / 2));
+		if (firmware_has_feature(FW_FEATURE_LPAR))
+			crashk_res.start = ppc64_rma_size / 2;
+		else
+			crashk_res.start = min(0x8000000ULL, (ppc64_rma_size / 2));
 #else
 		crashk_res.start = KDUMP_KERNELBASE;
 #endif