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

From: Huang, Kai
Date: Thu Aug 11 2022 - 06:08:35 EST


On Wed, 2022-08-10 at 17:59 -0700, Daniel Sneddon wrote:
> On 8/10/22 17:38, Thomas Gleixner wrote:
> > On Wed, Aug 10 2022 at 17:01, Daniel Sneddon wrote:
> > > On 8/10/22 16:44, Dave Hansen wrote:
> > > > On 8/10/22 16:38, Daniel Sneddon wrote:
> > > > > >
> > > > > > config INTEL_TDX_GUEST
> > > > > > bool "Intel TDX (Trust Domain Extensions) - Guest Support"
> > > > > > depends on X86_64 && CPU_SUP_INTEL
> > > > > > depends on X86_X2APIC
> > > > > So I got some more input. SPR and newer will lock the APIC.
> > > >
> > > > Could you get a _little_ more clarity on this, please? Exactly how and
> > > > when will it be locked? What does the BIOS writer's guide say? Will
> > > > there be an explicit x2APIC lock option? Or, will it be implicitly
> > > > locked when SGX or TDX is enabled?
> > >
> > > The BIOS doesn't explicitly lock the APIC. The APIC will be locked if X2APIC
> > > mode is enabled when the BIOS does an MCHECK. X2APIC mode will be enabled if
> > > SGX or TDX are enabled. So when exactly does the BIOS do an MCHECK? That I'll
> > > have to get clarification on.
> >
> > Sorry, this is uncomprehensible word salad and none of this makes any
> > sense at all to me.
> >
> > Can you please explain this in comprehensible sentences understandable
> > by mere mortals?
>
> Basically if the BIOS is configured to enable SGX or TDX, it'll program the APIC
> to use x2APIC mode. At some point it'll have to run MCHECK (which is just an
> MSR write). When exactly the BIOS does this, I'm not sure. I've asked for
> clarification on that. At the point the BIOS does the MCHECK, if X2APIC mode is
> enabled, the ucode will set the LOCK bit, and any attempt after that to disable
> the APIC will result in the fault. Now, this assumes the BIOS will DTRT, and
> always enable x2APIC when SGX or TDX are enabled AND always perform the MCHECK,
> AND do those things in the right order. I'm not a BIOS guy so I have no idea
> where to even look to see if/where that is documented. I can certainly try to
> track that down if needed.
>
> I'm not sure if that's any clearer? I guess I could try some code:
>
> if (SGX_enabled() || TDX_enabled()
> set_x2apic_mode();
>
> .....
>
> MCHECK <-----At this point if x2APIC mode is on then the ucode set's the lock bit.
>
> .....
>
>
> Hope that helps.
>
>

Hi Daniel,

This is new to me. Could you also include me when you are seeking for internal
clarification (and documentation publication)?

--
Thanks,
-Kai