[kvm-unit-tests PATCH 4/4] x86: Extend ASM_TRY to handle #UD thrown by FEP-triggered emulator

[Michal Luczaj <mhal@rbox.co>]

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

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
---
While here, I've also took the opportunity to merge both 32 and 64-bit
versions of ASM_TRY() (.dc.a for .long and .quad), but perhaps there
were some reasons for not using .dc.a?

 lib/x86/desc.h | 11 +++++------
 x86/emulator.c |  4 ++--
 2 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/lib/x86/desc.h b/lib/x86/desc.h
index 2a285eb..99cc224 100644
--- a/lib/x86/desc.h
+++ b/lib/x86/desc.h
@@ -80,21 +80,20 @@ typedef struct  __attribute__((packed)) {
 	u16 iomap_base;
 } tss64_t;
 
-#ifdef __x86_64
 #define ASM_TRY(catch)			\
 	"movl $0, %%gs:4 \n\t"		\
 	".pushsection .data.ex \n\t"	\
-	".quad 1111f, " catch "\n\t"	\
+	".dc.a 1111f, " catch "\n\t"	\
 	".popsection \n\t"		\
 	"1111:"
-#else
-#define ASM_TRY(catch)			\
+
+#define ASM_TRY_PREFIXED(prefix, catch)	\
 	"movl $0, %%gs:4 \n\t"		\
 	".pushsection .data.ex \n\t"	\
-	".long 1111f, " catch "\n\t"	\
+	".dc.a 1111f, " catch "\n\t"	\
 	".popsection \n\t"		\
+	prefix "\n\t"			\
 	"1111:"
-#endif
 
 /*
  * selector     32-bit                        64-bit
diff --git a/x86/emulator.c b/x86/emulator.c
index df0bc49..d2a5302 100644
--- a/x86/emulator.c
+++ b/x86/emulator.c
@@ -900,8 +900,8 @@ static void test_illegal_lea(void)
 {
 	unsigned int vector;
 
-	asm volatile (ASM_TRY("1f")
-		      KVM_FEP ".byte 0x8d; .byte 0xc0\n\t"
+	asm volatile (ASM_TRY_PREFIXED(KVM_FEP, "1f")
+		      ".byte 0x8d; .byte 0xc0\n\t"
 		      "1:"
 		      : : : "memory", "eax");
 
-- 
2.37.1