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

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 13, 2023, Mathias Krause wrote:
> On 06.04.23 18:43, Sean Christopherson wrote:
> > On Thu, Apr 06, 2023, Mathias Krause wrote:
> >> On 06.04.23 05:02, Sean Christopherson wrote:
> >>> [...]
> >>> 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));
> >>
> >> Yeah, the latter. Verifying the error code is part of the test, so that
> >> should be preserved.
> >>
> >> The tests as written by me also ensure that an exception actually
> >> occurred, exactly one, actually. Maybe that should be accounted for in
> >> asm_safe*() as well?
> > 
> > That's accounted for, the ASM_TRY() machinery treats "0" as no exception (we
> > sacrified #DE for the greater good).
> 
> I overlooked the GS-relative MOVL in ASM_TRY() first, which, after some
> digging, turns out to be zeroing the per-cpu 'exception_data' member.
> Sneaky ;)

Heh, "sneaky" is a much more polite description than I would use.  I really don't
like using per-CPU data for excpetion fixup, but having to support 32-bit builds
means our options our limited.  E.g. KVM selftests is 64-bit only and so can use
r9-r11 to communicate with the exception handler without conflicting with instructions
that have hardcoded registers (testing SYSRET isn't exactly a priority).