KVM: selftests: Add and use a helper function for x86's LIDT

Implement a function for setting the IDT descriptor from the guest
code. Replace the existing lidt occurrences with calls to this function
as `lidt` is used in multiple places.

Signed-off-by: Ivan Orlov <iorlov@amazon.com>
Link: https://lore.kernel.org/r/20241217181458.68690-7-iorlov@amazon.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: iorlov-dev

tools/testing/selftests/kvm/include/x86/processor.h

@@ -569,6 +569,11 @@ static inline void set_cr4(uint64_t val)
 	__asm__ __volatile__("mov %0, %%cr4" : : "r" (val) : "memory");
 }
 
+static inline void set_idt(const struct desc_ptr *idt_desc)
+{
+	__asm__ __volatile__("lidt %0"::"m"(*idt_desc));
+}
+
 static inline u64 xgetbv(u32 index)
 {
 	u32 eax, edx;

tools/testing/selftests/kvm/set_memory_region_test.c

@@ -235,7 +235,7 @@ static void guest_code_delete_memory_region(void)
 	 * in the guest will never succeed, and so isn't an option.
 	 */
 	memset(&idt, 0, sizeof(idt));
-	__asm__ __volatile__("lidt %0" :: "m"(idt));
+	set_idt(&idt);
 
 	GUEST_SYNC(0);
 

tools/testing/selftests/kvm/x86/sev_smoke_test.c

@@ -155,7 +155,7 @@ static void guest_shutdown_code(void)
 
 	/* Clobber the IDT so that #UD is guaranteed to trigger SHUTDOWN. */
 	memset(&idt, 0, sizeof(idt));
-	__asm__ __volatile__("lidt %0" :: "m"(idt));
+	set_idt(&idt);
 
 	__asm__ __volatile__("ud2");
 }