x86/sev: Make the #VC exception stacks part of the default stacks storage

The size of the exception stacks was increased by the commit in Fixes,
resulting in stack sizes greater than a page in size. The #VC exception
handling was only mapping the first (bottom) page, resulting in an
SEV-ES guest failing to boot.

Make the #VC exception stacks part of the default exception stacks
storage and allocate them with a CONFIG_AMD_MEM_ENCRYPT=y .config. Map
them only when a SEV-ES guest has been detected.

Rip out the custom VC stacks mapping and storage code.

 [ bp: Steal and adapt Tom's commit message. ]

Fixes: 7fae4c24a2 ("x86: Increase exception stack sizes")
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Tom Lendacky <thomas.lendacky@amd.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>
Link: https://lkml.kernel.org/r/YVt1IMjIs7pIZTRR@zn.tnic

arch/x86/include/asm/cpu_entry_area.h

@@ -10,6 +10,12 @@
 
 #ifdef CONFIG_X86_64
 
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+#define VC_EXCEPTION_STKSZ	EXCEPTION_STKSZ
+#else
+#define VC_EXCEPTION_STKSZ	0
+#endif
+
 /* Macro to enforce the same ordering and stack sizes */
 #define ESTACKS_MEMBERS(guardsize, optional_stack_size)		\
 	char	DF_stack_guard[guardsize];			\
@@ -28,7 +34,7 @@
 
 /* The exception stacks' physical storage. No guard pages required */
 struct exception_stacks {
-	ESTACKS_MEMBERS(0, 0)
+	ESTACKS_MEMBERS(0, VC_EXCEPTION_STKSZ)
 };
 
 /* The effective cpu entry area mapping with guard pages. */

arch/x86/kernel/sev.c

@@ -46,16 +46,6 @@ static struct ghcb __initdata *boot_ghcb;
 struct sev_es_runtime_data {
 	struct ghcb ghcb_page;
 
-	/* Physical storage for the per-CPU IST stack of the #VC handler */
-	char ist_stack[EXCEPTION_STKSZ] __aligned(PAGE_SIZE);
-
-	/*
-	 * Physical storage for the per-CPU fall-back stack of the #VC handler.
-	 * The fall-back stack is used when it is not safe to switch back to the
-	 * interrupted stack in the #VC entry code.
-	 */
-	char fallback_stack[EXCEPTION_STKSZ] __aligned(PAGE_SIZE);
-
 	/*
 	 * Reserve one page per CPU as backup storage for the unencrypted GHCB.
 	 * It is needed when an NMI happens while the #VC handler uses the real
@@ -99,27 +89,6 @@ DEFINE_STATIC_KEY_FALSE(sev_es_enable_key);
 /* Needed in vc_early_forward_exception */
 void do_early_exception(struct pt_regs *regs, int trapnr);
 
-static void __init setup_vc_stacks(int cpu)
-{
-	struct sev_es_runtime_data *data;
-	struct cpu_entry_area *cea;
-	unsigned long vaddr;
-	phys_addr_t pa;
-
-	data = per_cpu(runtime_data, cpu);
-	cea  = get_cpu_entry_area(cpu);
-
-	/* Map #VC IST stack */
-	vaddr = CEA_ESTACK_BOT(&cea->estacks, VC);
-	pa    = __pa(data->ist_stack);
-	cea_set_pte((void *)vaddr, pa, PAGE_KERNEL);
-
-	/* Map VC fall-back stack */
-	vaddr = CEA_ESTACK_BOT(&cea->estacks, VC2);
-	pa    = __pa(data->fallback_stack);
-	cea_set_pte((void *)vaddr, pa, PAGE_KERNEL);
-}
-
 static __always_inline bool on_vc_stack(struct pt_regs *regs)
 {
 	unsigned long sp = regs->sp;
@@ -787,7 +756,6 @@ void __init sev_es_init_vc_handling(void)
 	for_each_possible_cpu(cpu) {
 		alloc_runtime_data(cpu);
 		init_ghcb(cpu);
-		setup_vc_stacks(cpu);
 	}
 
 	sev_es_setup_play_dead();

arch/x86/mm/cpu_entry_area.c

@@ -110,6 +110,13 @@ static void __init percpu_setup_exception_stacks(unsigned int cpu)
 	cea_map_stack(NMI);
 	cea_map_stack(DB);
 	cea_map_stack(MCE);
+
+	if (IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT)) {
+		if (cc_platform_has(CC_ATTR_GUEST_STATE_ENCRYPT)) {
+			cea_map_stack(VC);
+			cea_map_stack(VC2);
+		}
+	}
 }
 #else
 static inline void percpu_setup_exception_stacks(unsigned int cpu)