arm64: kdump: Support crashkernel=X fall back to reserve region above DMA zones

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2112877
Upstream Status: linux tree
Tested: on ampere-mtjade-altra-03.khw4.lab.eng.bos.redhat.com

commit a9ae89d
Author: Zhen Lei <thunder.leizhen@huawei.com>
Date:   Wed Nov 16 20:10:44 2022 +0800

    arm64: kdump: Support crashkernel=X fall back to reserve region above DMA zones

    For crashkernel=X without '@offset', select a region within DMA zones
    first, and fall back to reserve region above DMA zones. This allows
    users to use the same configuration on multiple platforms.

    Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
    Acked-by: Baoquan He <bhe@redhat.com>
    Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
    Link: https://lore.kernel.org/r/20221116121044.1690-3-thunder.leizhen@huawei.com
    Signed-off-by: Will Deacon <will@kernel.org>

Signed-off-by: Pingfan Liu <piliu@redhat.com>

Author: Pingfan Liu

Documentation/admin-guide/kernel-parameters.txt

@@ -789,7 +789,7 @@
 			memory region [offset, offset + size] for that kernel
 			image. If '@offset' is omitted, then a suitable offset
 			is selected automatically.
-			[KNL, X86-64] Select a region under 4G first, and
+			[KNL, X86-64, ARM64] Select a region under 4G first, and
 			fall back to reserve region above 4G when '@offset'
 			hasn't been specified.
 			See Documentation/admin-guide/kdump/kdump.rst for further details.

arch/arm64/mm/init.c

@@ -132,6 +132,7 @@ static void __init reserve_crashkernel(void)
 	unsigned long long crash_max = CRASH_ADDR_LOW_MAX;
 	char *cmdline = boot_command_line;
 	int ret;
+	bool fixed_base = false;
 
 	if (!IS_ENABLED(CONFIG_KEXEC_CORE))
 		return;
@@ -163,12 +164,26 @@ static void __init reserve_crashkernel(void)
 	crash_size = PAGE_ALIGN(crash_size);
 
 	/* User specifies base address explicitly. */
-	if (crash_base)
+	if (crash_base) {
+		fixed_base = true;
 		crash_max = crash_base + crash_size;
+	}
 
+retry:
 	crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN,
 					       crash_base, crash_max);
 	if (!crash_base) {
+		/*
+		 * If the first attempt was for low memory, fall back to
+		 * high memory, the minimum required low memory will be
+		 * reserved later.
+		 */
+		if (!fixed_base && (crash_max == CRASH_ADDR_LOW_MAX)) {
+			crash_max = CRASH_ADDR_HIGH_MAX;
+			crash_low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
+			goto retry;
+		}
+
 		pr_warn("cannot allocate crashkernel (size:0x%llx)\n",
 			crash_size);
 		return;