[kvm-unit-tests PATCH v3 4/7] x86: Add helper to detect if forced emulation prefix is available

[Sean Christopherson <seanjc@google.com>]

Add a helper to detect whether or not KVM's forced emulation prefix is
available.  Use the helper to replace equivalent functionality in the
emulator test.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 lib/x86/desc.h | 14 ++++++++++++++
 x86/emulator.c | 15 +--------------
 2 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/lib/x86/desc.h b/lib/x86/desc.h
index 8f708fd..c023b93 100644
--- a/lib/x86/desc.h
+++ b/lib/x86/desc.h
@@ -106,6 +106,20 @@ typedef struct  __attribute__((packed)) {
 #define KVM_FEP "ud2; .byte 'k', 'v', 'm';"
 #define ASM_TRY_FEP(catch) __ASM_TRY(KVM_FEP, catch)
 
+static inline bool is_fep_available(void)
+{
+	/*
+	 * Use the non-FEP ASM_TRY() as KVM will inject a #UD on the prefix
+	 * itself if forced emulation is not available.
+	 */
+	asm goto(ASM_TRY("%l[fep_unavailable]")
+		 KVM_FEP "nop\n\t"
+		 ::: "memory" : fep_unavailable);
+	return true;
+fep_unavailable:
+	return false;
+}
+
 /*
  * selector     32-bit                        64-bit
  * 0x00         NULL descriptor               NULL descriptor
diff --git a/x86/emulator.c b/x86/emulator.c
index 6dc88f1..e1272a6 100644
--- a/x86/emulator.c
+++ b/x86/emulator.c
@@ -17,10 +17,6 @@
 
 static int exceptions;
 
-/* Forced emulation prefix, used to invoke the emulator unconditionally.  */
-#define KVM_FEP_LENGTH 5
-static int fep_available = 1;
-
 struct regs {
 	u64 rax, rbx, rcx, rdx;
 	u64 rsi, rdi, rsp, rbp;
@@ -1121,12 +1117,6 @@ static void test_illegal_movbe(void)
 	handle_exception(UD_VECTOR, 0);
 }
 
-static void record_no_fep(struct ex_regs *regs)
-{
-	fep_available = 0;
-	regs->rip += KVM_FEP_LENGTH;
-}
-
 int main(void)
 {
 	void *mem;
@@ -1136,9 +1126,6 @@ int main(void)
 	unsigned long t1, t2;
 
 	setup_vm();
-	handle_exception(UD_VECTOR, record_no_fep);
-	asm(KVM_FEP "nop");
-	handle_exception(UD_VECTOR, 0);
 
 	mem = alloc_vpages(2);
 	install_page((void *)read_cr3(), IORAM_BASE_PHYS, mem);
@@ -1190,7 +1177,7 @@ int main(void)
 	test_ltr(mem);
 	test_cmov(mem);
 
-	if (fep_available) {
+	if (is_fep_available()) {
 		test_mmx_movq_mf(mem);
 		test_movabs(mem);
 		test_smsw_reg(mem);
-- 
2.37.1.559.g78731f0fdb-goog