[kvm-unit-tests PATCH v2 3/5] x86: Introduce ASM_TRY_FEP() to handle exceptions thrown by FEP-triggered emulator

[Michal Luczaj <mhal@rbox.co>]

From: Paolo Bonzini <pbonzini@redhat.com>

TRY_ASM() mishandles exceptions thrown by the forced-emulation-triggered
emulator. While the faulting address stored in the exception table points
at forced emulation prefix, when an exceptions comes, RIP is 5 bytes
(size of KVM_FEP) ahead and the exception ends up unhandled.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
---
For the sake of completeness, I've added `prefix` to 32-bit __TRY_ASM()
as well.

 lib/x86/desc.h | 8 ++++++--
 x86/emulator.c | 1 +
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/lib/x86/desc.h b/lib/x86/desc.h
index 2a285eb..8854fcc 100644
--- a/lib/x86/desc.h
+++ b/lib/x86/desc.h
@@ -81,21 +81,25 @@ typedef struct  __attribute__((packed)) {
 } tss64_t;
 
 #ifdef __x86_64
-#define ASM_TRY(catch)			\
+#define __ASM_TRY(prefix, catch)	\
 	"movl $0, %%gs:4 \n\t"		\
 	".pushsection .data.ex \n\t"	\
 	".quad 1111f, " catch "\n\t"	\
 	".popsection \n\t"		\
+	prefix				\
 	"1111:"
 #else
-#define ASM_TRY(catch)			\
+#define __ASM_TRY(prefix, catch)	\
 	"movl $0, %%gs:4 \n\t"		\
 	".pushsection .data.ex \n\t"	\
 	".long 1111f, " catch "\n\t"	\
 	".popsection \n\t"		\
+	prefix				\
 	"1111:"
 #endif
 
+#define ASM_TRY(catch) __ASM_TRY("", catch)
+
 /*
  * selector     32-bit                        64-bit
  * 0x00         NULL descriptor               NULL descriptor
diff --git a/x86/emulator.c b/x86/emulator.c
index 96d3ccd..0eb7c1a 100644
--- a/x86/emulator.c
+++ b/x86/emulator.c
@@ -19,6 +19,7 @@ static int exceptions;
 
 /* Forced emulation prefix, used to invoke the emulator unconditionally. */
 #define KVM_FEP "ud2; .byte 'k', 'v', 'm';"
+#define ASM_TRY_FEP(catch) __ASM_TRY(KVM_FEP, catch)
 
 struct regs {
 	u64 rax, rbx, rcx, rdx;
-- 
2.37.1