Re: [kvm-unit-tests PATCH v2] x86: Test illegal LEA handling

[Michal Luczaj <mhal@rbox.co>]

On 8/1/22 18:44, Sean Christopherson wrote:
> On Sun, Jul 31, 2022, Michal Luczaj wrote:
>> +{
>> +	exceptions = 0;
>> +	handle_exception(UD_VECTOR, illegal_lea_handler);
> 
> No need to use a custom handler (ignore any patterns in emulator.c that suggest
> it's "mandatory", emulator is one of the oldest test).  ASM_TRY() can handle all
> of this without any globals.
> ...
> static void test_illegal_lea(void)
> {
> 	unsigned int vector;
> 
> 	asm volatile (ASM_TRY("1f")
> 		      KVM_FEP ".byte 0x8d; .byte 0xc0\n\t"
> 		      "1:"
> 		      : : : "memory", "eax");
> 
> 	vector = exception_vector();
> 	report(vector == UD_VECTOR,
> 	       "Wanted #UD on LEA with /reg, got vector = %d", vector);
> }

I must be missing something important. There is
`handle_exception(UD_VECTOR, 0)` early in `main()` which simply undoes
`handle_exception(6, check_exception_table)` set by `setup_idt()`. If
there's no more exception table walk for #UD, `ASM_TRY` alone can't
possibly work, am I corrent?

If so, am I supposed to restore the `check_exception_table()` handler? Or
maybe using `test_for_exception()` would be more elegant:

static void illegal_lea(void *unused)
{
	asm volatile(KVM_FEP ".byte 0x8d, 0xc0" : : : "memory", "eax");
}

static void test_illegal_lea(void)
{
	bool fault;

	fault = test_for_exception(UD_VECTOR, &illegal_lea, NULL);
	report(fault, "Wanted #UD on LEA with /reg");
}

Thanks for hints,
Michal