x86/boot/e820: Rename early_reserve_e820() to e820__memblock_alloc() and document it

early_reserve_e820() is an early hack for kexec that does a limited fixup of the
mptable and passes it to the kexec kernel as if it was the real thing.

For this it needs to allocate memory - but no memory allocator is available yet
beyond the memblock allocator, so early_reserve_e820() is really a wrapper
around memblock_alloc() plus a hack to update the e820_table_firmware entries.

The name 'reserve' is really a bit of a misnomer, as 'reserved' memory typically
means memory completely inaccessible to the kernel - while here what we want to do
is a special RAM allocation for our own purposes and insert that as RAM_RESERVED.

Rename the function to e820__memblock_alloc_reserved() to better signal this dual
purpose, plus document it better, which was omitted when it was merged. The barely
comprehensible and cryptic comment:

  /*
   * pre allocated 4k and reserved it in memblock and e820_table_firmware
   */
  u64 __init e820__memblock_alloc_reserved(u64 size, u64 align)

... does not count as documentation, replace it with:

  /*
   * Allocate the requested number of bytes with the requsted alignment
   * and return (the physical address) to the caller. Also register this
   * range in the 'firmware' E820 table.
   *
   * This allows kexec to fake a new mptable, as if it came from the real
   * system.
   */
  u64 __init e820__memblock_alloc_reserved(u64 size, u64 align)

No change in functionality.

Cc: Alex Thorlton <athorlton@sgi.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Huang, Ying <ying.huang@intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Jackson <pj@sgi.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/e820/api.h

@@ -20,7 +20,7 @@ extern void e820_setup_gap(void);
 extern void e820__memory_setup_extended(u64 phys_addr, u32 data_len);
 extern unsigned long e820_end_of_ram_pfn(void);
 extern unsigned long e820_end_of_low_ram_pfn(void);
-extern u64  early_reserve_e820(u64 sizet, u64 align);
+extern u64  e820__memblock_alloc_reserved(u64 sizet, u64 align);
 extern void e820__memblock_setup(void);
 extern void e820_reserve_setup_data(void);
 extern void e820__finish_early_params(void);

arch/x86/include/asm/mpspec.h

@@ -64,7 +64,7 @@ static inline void find_smp_config(void)
 }
 
 #ifdef CONFIG_X86_MPPARSE
-extern void early_reserve_e820_mpc_new(void);
+extern void e820__memblock_alloc_reserved_mpc_new(void);
 extern int enable_update_mptable;
 extern int default_mpc_apic_id(struct mpc_cpu *m);
 extern void default_smp_read_mpc_oem(struct mpc_table *mpc);
@@ -76,7 +76,7 @@ extern void default_mpc_oem_bus_info(struct mpc_bus *m, char *str);
 extern void default_find_smp_config(void);
 extern void default_get_smp_config(unsigned int early);
 #else
-static inline void early_reserve_e820_mpc_new(void) { }
+static inline void e820__memblock_alloc_reserved_mpc_new(void) { }
 #define enable_update_mptable 0
 #define default_mpc_apic_id NULL
 #define default_smp_read_mpc_oem NULL

arch/x86/kernel/e820.c

@@ -734,16 +734,21 @@ core_initcall(e820_mark_nvs_memory);
 #endif
 
 /*
- * pre allocated 4k and reserved it in memblock and e820_table_firmware
+ * Allocate the requested number of bytes with the requsted alignment
+ * and return (the physical address) to the caller. Also register this
+ * range in the 'firmware' E820 table as a reserved range.
+ *
+ * This allows kexec to fake a new mptable, as if it came from the real
+ * system.
  */
-u64 __init early_reserve_e820(u64 size, u64 align)
+u64 __init e820__memblock_alloc_reserved(u64 size, u64 align)
 {
 	u64 addr;
 
 	addr = __memblock_alloc_base(size, align, MEMBLOCK_ALLOC_ACCESSIBLE);
 	if (addr) {
 		e820_update_range_firmware(addr, size, E820_RAM, E820_RESERVED);
-		pr_info("e820: update e820_table_firmware for early_reserve_e820\n");
+		pr_info("e820: update e820_table_firmware for e820__memblock_alloc_reserved()\n");
 		update_e820_table_firmware();
 	}
 

arch/x86/kernel/mpparse.c

@@ -826,10 +826,10 @@ static int __init parse_alloc_mptable_opt(char *p)
 }
 early_param("alloc_mptable", parse_alloc_mptable_opt);
 
-void __init early_reserve_e820_mpc_new(void)
+void __init e820__memblock_alloc_reserved_mpc_new(void)
 {
 	if (enable_update_mptable && alloc_mptable)
-		mpc_new_phys = early_reserve_e820(mpc_new_length, 4);
+		mpc_new_phys = e820__memblock_alloc_reserved(mpc_new_length, 4);
 }
 
 static int __init update_mp_table(void)

arch/x86/kernel/setup.c

@@ -1112,7 +1112,7 @@ void __init setup_arch(char **cmdline_p)
 	}
 
 	/* preallocate 4k for mptable mpc */
-	early_reserve_e820_mpc_new();
+	e820__memblock_alloc_reserved_mpc_new();
 
 #ifdef CONFIG_X86_CHECK_BIOS_CORRUPTION
 	setup_bios_corruption_check();