Re: [PATCH] x86/apic: Don't disable x2APIC if locked

[Thomas Gleixner]

On Wed, Aug 10 2022 at 12:40, Daniel Sneddon wrote:
> On 8/10/22 11:52, Dave Hansen wrote:
>> On 8/10/22 11:30, Daniel Sneddon wrote:
>>>> If it's going to be on one server CPU that's coming out in ten years,
>>>> then we can hold off.
>>> SPR will certainly be sooner than 10 years, and with distros running on LTS
>>> kernels, it is probably worth backporting.  Since this isn't a bug fix (not a sw
>>> bug anyway), is this something I can just CC stable or is there a better way to
>>> say "Yes, this is a new feature, BUT, you really want it anyway"?
>> 
>> It it going to be *forced* on those SPR system?  In other words, is it a
>> little BIOS switch that users can flip?  Is there any non-kernel
>> workaround if a user hits this, or is the entire burden of this going to
>> be foisted on the kernel?
> It won't be forced, BUT, certain features won't be available if the APIC isn't
> locked.  According to the documentation SGX and TDX won't be available if you
> don't lock the APIC.  So, yes, you don't have to fix it in the kernel, but you
> may lose access to features if you don't.
>> 
>> The worst case scenario would be if a user has managed to compile a
>> CONFIG_X86_X2APIC=n kernel and is happily running along until they get a
>> microcode update, reboot and can't boot any more.
> That _shouldn't_ happen as it is only on new hardware, so a ucode update
> _shouldn't_ cause a crash.

Only on new hardware? The lock mechanism has to be available on _all_
affected systems which support SGX. See

 https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/resources/intel-sgx-software-and-tcb-recovery-guidance.html

That means at some point in the future the locked x2APIC will be a
prerequisite for attestating a SGX enclave independent of SPR.

So this affects already deployed systems and we have to

  - backport the x2apic lock changes

  - provide proper diagnostics

  - make SGX and TDX depend on X2APIC support

There is not much we can do about an older kernel which fails to boot or
explodes once a BIOS update locks down X2APIC.

Thanks,

        tglx