Messages in this thread | | | From | "Huang, Kai" <> | Subject | Re: [PATCH] x86/apic: Don't disable x2APIC if locked | Date | Thu, 11 Aug 2022 10:08:22 +0000 |
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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
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