[RFC 3/7] x86/fixmap: Generalize the GDT fixmap mechanism

[Andy Lutomirski <luto@kernel.org>]

Currently, the GDT is an ad-hoc array of pages, one per CPU, in the
fixmap.  Generalize it to be an array of a new struct cpu_entry_area
so that we can cleanly add new things to it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/include/asm/desc.h   |  9 +--------
 arch/x86/include/asm/fixmap.h | 36 ++++++++++++++++++++++++++++++++++--
 arch/x86/kernel/cpu/common.c  | 14 +++++++-------
 arch/x86/xen/mmu_pv.c         |  2 +-
 4 files changed, 43 insertions(+), 18 deletions(-)

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index 02a414cda30f..92ad5c354d11 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -59,17 +59,10 @@ static inline struct desc_struct *get_current_gdt_rw(void)
 	return this_cpu_ptr(&gdt_page)->gdt;
 }
 
-/* Get the fixmap index for a specific processor */
-static inline unsigned int get_cpu_gdt_ro_index(int cpu)
-{
-	return FIX_GDT_REMAP_END - cpu;
-}
-
 /* Provide the fixmap address of the remapped GDT */
 static inline struct desc_struct *get_cpu_gdt_ro(int cpu)
 {
-	unsigned int idx = get_cpu_gdt_ro_index(cpu);
-	return (struct desc_struct *)__fix_to_virt(idx);
+	return (struct desc_struct *)&get_cpu_entry_area(cpu)->gdt;
 }
 
 /* Provide the current read-only GDT */
diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
index dcd9fb55e679..fbc9b7f4e35e 100644
--- a/arch/x86/include/asm/fixmap.h
+++ b/arch/x86/include/asm/fixmap.h
@@ -44,6 +44,17 @@ extern unsigned long __FIXADDR_TOP;
 			 PAGE_SIZE)
 #endif
 
+/*
+ * cpu_entry_area is a percpu region in the fixmap that contains things
+ * need by the CPU and early entry/exit code.  Real types aren't used here
+ * to avoid circular header dependencies.
+ */
+struct cpu_entry_area
+{
+	char gdt[PAGE_SIZE];
+};
+
+#define CPU_ENTRY_AREA_PAGES (sizeof(struct cpu_entry_area) / PAGE_SIZE)
 
 /*
  * Here we define all the compile-time 'special' virtual
@@ -101,8 +112,8 @@ enum fixed_addresses {
 	FIX_LNW_VRTC,
 #endif
 	/* Fixmap entries to remap the GDTs, one per processor. */
-	FIX_GDT_REMAP_BEGIN,
-	FIX_GDT_REMAP_END = FIX_GDT_REMAP_BEGIN + NR_CPUS - 1,
+	FIX_CPU_ENTRY_AREA_TOP,
+	FIX_CPU_ENTRY_AREA_BOTTOM = FIX_CPU_ENTRY_AREA_TOP + (CPU_ENTRY_AREA_PAGES * NR_CPUS) - 1,
 
 	__end_of_permanent_fixed_addresses,
 
@@ -185,5 +196,26 @@ void __init *early_memremap_decrypted_wp(resource_size_t phys_addr,
 void __early_set_fixmap(enum fixed_addresses idx,
 			phys_addr_t phys, pgprot_t flags);
 
+static inline unsigned int __get_cpu_entry_area_page_index(int cpu, int page)
+{
+	BUILD_BUG_ON(sizeof(struct cpu_entry_area) % PAGE_SIZE != 0);
+
+	return FIX_CPU_ENTRY_AREA_BOTTOM - cpu*CPU_ENTRY_AREA_PAGES - page;
+}
+
+#define __get_cpu_entry_area_offset_index(cpu, offset) ({		\
+	BUILD_BUG_ON(offset % PAGE_SIZE != 0);				\
+	__get_cpu_entry_area_page_index(cpu, offset / PAGE_SIZE);	\
+	})
+
+#define get_cpu_entry_area_index(cpu, field)				\
+	__get_cpu_entry_area_offset_index((cpu), offsetof(struct cpu_entry_area, field))
+
+static inline struct cpu_entry_area *get_cpu_entry_area(int cpu)
+{
+	return (struct cpu_entry_area *)
+		__fix_to_virt(__get_cpu_entry_area_page_index(cpu, 0));
+}
+
 #endif /* !__ASSEMBLY__ */
 #endif /* _ASM_X86_FIXMAP_H */
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 22f542170198..2cb394dc4153 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -466,12 +466,12 @@ void load_percpu_segment(int cpu)
 	load_stack_canary_segment();
 }
 
-/* Setup the fixmap mapping only once per-processor */
-static inline void setup_fixmap_gdt(int cpu)
+/* Setup the fixmap mappings only once per-processor */
+static inline void setup_cpu_entry_area(int cpu)
 {
 #ifdef CONFIG_X86_64
 	/* On 64-bit systems, we use a read-only fixmap GDT. */
-	pgprot_t prot = PAGE_KERNEL_RO;
+	pgprot_t gdt_prot = PAGE_KERNEL_RO;
 #else
 	/*
 	 * On native 32-bit systems, the GDT cannot be read-only because
@@ -482,11 +482,11 @@ static inline void setup_fixmap_gdt(int cpu)
 	 * On Xen PV, the GDT must be read-only because the hypervisor requires
 	 * it.
 	 */
-	pgprot_t prot = boot_cpu_has(X86_FEATURE_XENPV) ?
+	pgprot_t gdt_prot = boot_cpu_has(X86_FEATURE_XENPV) ?
 		PAGE_KERNEL_RO : PAGE_KERNEL;
 #endif
 
-	__set_fixmap(get_cpu_gdt_ro_index(cpu), get_cpu_gdt_paddr(cpu), prot);
+	__set_fixmap(get_cpu_entry_area_index(cpu, gdt), get_cpu_gdt_paddr(cpu), gdt_prot);
 }
 
 /* Load the original GDT from the per-cpu structure */
@@ -1589,7 +1589,7 @@ void cpu_init(void)
 	if (is_uv_system())
 		uv_cpu_init();
 
-	setup_fixmap_gdt(cpu);
+	setup_cpu_entry_area(cpu);
 	load_fixmap_gdt(cpu);
 }
 
@@ -1651,7 +1651,7 @@ void cpu_init(void)
 
 	fpu__init_cpu();
 
-	setup_fixmap_gdt(cpu);
+	setup_cpu_entry_area(cpu);
 	load_fixmap_gdt(cpu);
 }
 #endif
diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 71495f1a86d7..a04157699d3b 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -2311,7 +2311,7 @@ static void xen_set_fixmap(unsigned idx, phys_addr_t phys, pgprot_t prot)
 #endif
 	case FIX_TEXT_POKE0:
 	case FIX_TEXT_POKE1:
-	case FIX_GDT_REMAP_BEGIN ... FIX_GDT_REMAP_END:
+	case FIX_CPU_ENTRY_AREA_TOP ... FIX_CPU_ENTRY_AREA_BOTTOM:
 		/* All local page mappings */
 		pte = pfn_pte(phys, prot);
 		break;
-- 
2.13.6