x86: Support kmap_local() forced debugging

kmap_local() and related interfaces are NOOPs on 64bit and only create
temporary fixmaps for highmem pages on 32bit. That means the test coverage
for this code is pretty small.

CONFIG_KMAP_LOCAL can be enabled independent from CONFIG_HIGHMEM, which
allows to provide support for enforced kmap_local() debugging even on
64bit.

For 32bit the support is unconditional, for 64bit it's only supported when
CONFIG_NR_CPUS <= 4096 as supporting it for 8192 CPUs would require to set
up yet another fixmap PGT.

If CONFIG_KMAP_LOCAL_FORCE_DEBUG is enabled then kmap_local()/kmap_atomic()
will use the temporary fixmap mapping path.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20201118204007.169209557@linutronix.de

arch/x86/Kconfig

@@ -93,6 +93,7 @@ config X86
 	select ARCH_SUPPORTS_ACPI
 	select ARCH_SUPPORTS_ATOMIC_RMW
 	select ARCH_SUPPORTS_NUMA_BALANCING	if X86_64
+	select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP	if NR_CPUS <= 4096
 	select ARCH_USE_BUILTIN_BSWAP
 	select ARCH_USE_QUEUED_RWLOCKS
 	select ARCH_USE_QUEUED_SPINLOCKS

arch/x86/include/asm/fixmap.h

@@ -14,13 +14,20 @@
 #ifndef _ASM_X86_FIXMAP_H
 #define _ASM_X86_FIXMAP_H
 
+#include <asm/kmap_size.h>
+
 /*
  * Exposed to assembly code for setting up initial page tables. Cannot be
  * calculated in assembly code (fixmap entries are an enum), but is sanity
  * checked in the actual fixmap C code to make sure that the fixmap is
  * covered fully.
  */
-#define FIXMAP_PMD_NUM	2
+#ifndef CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP
+# define FIXMAP_PMD_NUM	2
+#else
+# define KM_PMDS	(KM_MAX_IDX * ((CONFIG_NR_CPUS + 511) / 512))
+# define FIXMAP_PMD_NUM (KM_PMDS + 2)
+#endif
 /* fixmap starts downwards from the 507th entry in level2_fixmap_pgt */
 #define FIXMAP_PMD_TOP	507
 
@@ -31,7 +38,6 @@
 #include <asm/pgtable_types.h>
 #ifdef CONFIG_X86_32
 #include <linux/threads.h>
-#include <asm/kmap_size.h>
 #else
 #include <uapi/asm/vsyscall.h>
 #endif
@@ -92,7 +98,7 @@ enum fixed_addresses {
 	FIX_IO_APIC_BASE_0,
 	FIX_IO_APIC_BASE_END = FIX_IO_APIC_BASE_0 + MAX_IO_APICS - 1,
 #endif
-#ifdef CONFIG_X86_32
+#ifdef CONFIG_KMAP_LOCAL
 	FIX_KMAP_BEGIN,	/* reserved pte's for temporary kernel mappings */
 	FIX_KMAP_END = FIX_KMAP_BEGIN + (KM_MAX_IDX * NR_CPUS) - 1,
 #ifdef CONFIG_PCI_MMCONFIG

arch/x86/include/asm/pgtable_64_types.h

@@ -143,7 +143,11 @@ extern unsigned int ptrs_per_p4d;
 
 #define MODULES_VADDR		(__START_KERNEL_map + KERNEL_IMAGE_SIZE)
 /* The module sections ends with the start of the fixmap */
-#define MODULES_END		_AC(0xffffffffff000000, UL)
+#ifndef CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP
+# define MODULES_END		_AC(0xffffffffff000000, UL)
+#else
+# define MODULES_END		_AC(0xfffffffffe000000, UL)
+#endif
 #define MODULES_LEN		(MODULES_END - MODULES_VADDR)
 
 #define ESPFIX_PGD_ENTRY	_AC(-2, UL)