Re: [v3 03/15] Add cmpxchg16b support for x86-64

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 08/07/2015 09:12, Jan Beulich wrote:
>
>>>
>>>> +{
>>>> +    uint128_t prev;
>>>> +
>>>> +    ASSERT(cpu_has_cx16);
>>> Given that if this assertion were to fail, cmpxchg16b would fail with
>>> #UD, I would hand-code a asm_fixup section which in turn panics.  This
>>> avoids a situation where non-debug builds could die with an unqualified
>>> #UD exception.
>> Is there an existing way to panic the hypervisor in assembler code, I
>> don't find it, it would be appreciated if you can point it out.

When I asked for this, I was thinking of having an assertion frame with
the cmpxchg16b instruction in the place of the regular ud2a.  This way,
if it were to failed with #UD, there is a more useful error message.

However, there is no easy way of doing this at the moment, and it is an
obscure set of circumstances, so probably not worth the hassle.

> I'm not convinced such a #UD would be a significant problem: Looking
> at the disassembly will show the cause right away. The out of line
> ud2-s in some of VMX'es inline assembly wrappers are far worse.

Unqualified #UDs are harder to debug than qualified ones, and I have an
annoying habit of hitting them.  In some copious free time, I want to
continue the work started with c/s 0a3e27e and 881d6bf.  git grep
suggests there isn't actually too much to fix up in this regard.

~Andrew