From: Joerg Roedel <redacted>

Allocate and map an IST stack and an additional fall-back stack for
the #VC handler.  The memory for the stacks is allocated only when
SEV-ES is active.

The #VC handler needs to use an IST stack because it could be raised
from kernel space with unsafe stack, e.g. in the SYSCALL entry path.

Since the #VC exception can be nested, the #VC handler switches back to
the interrupted stack when entered from kernel space. If switching back
is not possible the fall-back stack is used.

Signed-off-by: Joerg Roedel <redacted>
Link: https://lore.kernel.org/r/20200724160336.5435-45-joro@8bytes.org (local)
---
 arch/x86/include/asm/cpu_entry_area.h | 33 +++++++++++++++++----------
 arch/x86/include/asm/page_64_types.h  |  1 +
 arch/x86/kernel/cpu/common.c          |  2 ++
 arch/x86/kernel/dumpstack_64.c        |  8 +++++--
 arch/x86/kernel/sev-es.c              | 33 +++++++++++++++++++++++++++
 5 files changed, 63 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/cpu_entry_area.h b/arch/x86/include/asm/cpu_entry_area.h
index 8902fdb7de13..f87e4c0c16f4 100644
--- a/arch/x86/include/asm/cpu_entry_area.h
+++ b/arch/x86/include/asm/cpu_entry_area.h

@@ -11,25 +11,29 @@
 #ifdef CONFIG_X86_64
 
 /* Macro to enforce the same ordering and stack sizes */
-#define ESTACKS_MEMBERS(guardsize)		\
-	char	DF_stack_guard[guardsize];	\
-	char	DF_stack[EXCEPTION_STKSZ];	\
-	char	NMI_stack_guard[guardsize];	\
-	char	NMI_stack[EXCEPTION_STKSZ];	\
-	char	DB_stack_guard[guardsize];	\
-	char	DB_stack[EXCEPTION_STKSZ];	\
-	char	MCE_stack_guard[guardsize];	\
-	char	MCE_stack[EXCEPTION_STKSZ];	\
-	char	IST_top_guard[guardsize];	\
+#define ESTACKS_MEMBERS(guardsize, optional_stack_size)		\
+	char	DF_stack_guard[guardsize];			\
+	char	DF_stack[EXCEPTION_STKSZ];			\
+	char	NMI_stack_guard[guardsize];			\
+	char	NMI_stack[EXCEPTION_STKSZ];			\
+	char	DB_stack_guard[guardsize];			\
+	char	DB_stack[EXCEPTION_STKSZ];			\
+	char	MCE_stack_guard[guardsize];			\
+	char	MCE_stack[EXCEPTION_STKSZ];			\
+	char	VC_stack_guard[guardsize];			\
+	char	VC_stack[optional_stack_size];			\
+	char	VC2_stack_guard[guardsize];			\
+	char	VC2_stack[optional_stack_size];			\

+	char	IST_top_guard[guardsize];			\
 
 /* The exception stacks' physical storage. No guard pages required */
 struct exception_stacks {
-	ESTACKS_MEMBERS(0)
+	ESTACKS_MEMBERS(0, 0)
 };
 
 /* The effective cpu entry area mapping with guard pages. */
 struct cea_exception_stacks {
-	ESTACKS_MEMBERS(PAGE_SIZE)
+	ESTACKS_MEMBERS(PAGE_SIZE, EXCEPTION_STKSZ)
 };
 
 /*

@@ -40,6 +44,8 @@ enum exception_stack_ordering {
 	ESTACK_NMI,
 	ESTACK_DB,
 	ESTACK_MCE,
+	ESTACK_VC,
+	ESTACK_VC2,
 	N_EXCEPTION_STACKS
 };
 

@@ -139,4 +145,7 @@ static inline struct entry_stack *cpu_entry_stack(int cpu)
 #define __this_cpu_ist_top_va(name)					\
 	CEA_ESTACK_TOP(__this_cpu_read(cea_exception_stacks), name)
 
+#define __this_cpu_ist_bot_va(name)					\