kexec reboot fails with extra wbinvd introduced for AME SME

[Tom Lendacky]

On 1/17/2018 1:42 PM, Linus Torvalds wrote:
> On Tue, Jan 16, 2018 at 11:22 PM, Dave Young <dyoung at redhat.com> wrote:
>>
>> For the kexec reboot hang, if I remove the wbinvd in stop_this_cpu()
>> then kexec works fine. like this:
> 
> Honestly, I think we should apply that patch regardless.
> 
> Using 'wbinvd' should not be some "just because of random reasons".
> There are CPU's with errata on wbinvd, and the thing in general is
> slow and nasty.
> 
> Doing the wbinvd in a loop sounds even stranger.
> 
> If we're only doing it because of some SME issue, why isn't it
> dependent on SME? And why is it inside that loop at all?

My original patches did check for X86_FEATURE_SME and only do the
wbinvd if SME was supported (although still in the loop).  The general
consensus was to just do the wbinvd no matter what and so it is as it is
today.

It can probably be outside of the loop.  The issue I was seeing was
memory corruption from the stack when using halt() with paravirt ops
enabled.  So a native_halt() should be used.

> 
> Anyway, does it work for you if you just do the wbinvd() once, outside
> the loop? Admittedly the loop shouldn't actually loop (hlt with
> interrupts disabled), but who the hell knows.. Some of the errata
> around SME have been about machine check exceptions or something.

I think that should work as long as it's a native_wbinvd() call and it
can also be conditional on boot_cpu_has(X86_FEATURE_SME).

I'll do some testing.

Thanks,
Tom

> 
> See commit a68e5c94f7d3 ("x86, hotplug: Move WBINVD back outside the
> play_dead loop") for another example where wbinvd was inside a loop
> and apparently caused some odd issues.
> 
>               Linus
>