kexec reboot fails with extra wbinvd introduced for AME SME

[Tom Lendacky]

On 1/17/2018 8:29 PM, Dave Young wrote:
> On 01/17/18 at 06:14pm, Linus Torvalds wrote:
>> On Wed, Jan 17, 2018 at 5:47 PM, Dave Young <dyoung at redhat.com> wrote:
>>>
>>> It does not work with just once wbinvd(), and it only works with
>>> removing the wbinvd() for me.  Tom's new post works for me as well
>>> since my cpu is an Intel i5-4200U.
>>
>> Intriguing.
>>
>> It's not like the wbinvd really should be that much of a deal.
>>
>> I think Tom's patch is fine and should be applied, but it does worry
>> me a bit that even a single wbinvd makes that much of a difference for
>> you. There is very little logical reason I can think of that a wbinvd
>> should make any difference what-so-ever on an i5-4200U.
>>
>> I wonder if you have some system issues, and wbinvd just happens to
>> trigger them. But I think we do wbinvd before a suspend-to-RAM too
>> (it's "ACPI_FLUSH_CPU_CACHE()" in the ACPI code). And the dmr code
>> dioes "wbinvd_on_all_cpus()" which does a cross-call etc.
>>
>> Would you mind experimenting a bit with that wbinvd?
>>
>> In particular, what happens if you enable it (so it's not hidden by
>> the SME check), but you move it up to before interrupts are disabled?
> 
> Will play with it more.  Actually I found the hang seems happens
> in code of arch/x86/kernel/relocate_kernel_64.S, there is another
> wbinvd there as well.

The wbinvd in arch/x86/kernel/relocate_kernel_64.S is only performed if
SME is active, so that one won't be executed on an Intel chip.

Thanks,
Tom

> 
>>
>> I'm wondering if there is some issue with MCE generation and wbinvd
>> and whatever, and doing it when the CPU is down and interrupts are
>> disabled causes some system issue..
>>
>> Does anybody have any other ideas?
>>
>>                 Linus