Re: [kvm-unit-tests PATCH] x86/emulator: Test non-canonical memory access exceptions

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 15, 2023, Mathias Krause wrote:
> +static void test_reg_noncanonical(void)
> +{
> +	extern char nc_rsp_start, nc_rsp_end, nc_rbp_start, nc_rbp_end;
> +	extern char nc_rax_start, nc_rax_end;
> +	handler old_ss, old_gp;
> +
> +	old_ss = handle_exception(SS_VECTOR, advance_rip_and_note_exception);
> +	old_gp = handle_exception(GP_VECTOR, advance_rip_and_note_exception);
> +
> +	/* RAX based, should #GP(0) */
> +	exceptions = 0;
> +	rip_advance = &nc_rax_end - &nc_rax_start;
> +	asm volatile("nc_rax_start: orq $0, (%[msb]); nc_rax_end:\n\t"

Can't we use ASM_TRY() + exception_vector() + exception_error_code()?  Installing
a dedicated handler is (slowly) being phased out.  Even better, if you're willing
to take a dependency and/or wait a few weeks for my series to land[*], you should
be able to use asm_safe() to streamline this even further.

[*] https://lkml.kernel.org/r/20230406025117.738014-1-seanjc%40google.com


> +		     : : [msb]"a"(1ul << 63));

Use BIT_ULL().  Actually, scratch that, we have a NONCANONICAL macro.  It _probably_
won't matter, but who know what will happen with things like LAM and LASS.

And why hardcode use of RAX?  Won't any "r" constraint work?

E.g. I believe this can be something like:

	asm_safe_report_ex(GP_VECTOR, "orq $0, (%[noncanonical]), "r" (NONCANONICAL));
	report(!exception_error_code());

Or we could even add asm_safe_report_ex_ec(), e.g.

	asm_safe_report_ex_ec(GP_VECTOR, 0,
			      "orq $0, (%[noncanonical]), "r" (NONCANONICAL));