Re: [PATCH v15 22/23] x86/mce: Improve error log of kernel space TDX #MC due to erratum

Dave Hansen
2023-12-04T23:39:08Z

On 12/4/23 15:24, Huang, Kai wrote:
> On Mon, 2023-12-04 at 14:04 -0800, Hansen, Dave wrote:
...
> In ancient time KVM used to immediately enable VMX when it is loaded, but later
> it was changed to only enable VMX when there's active VM because of the above
> reason.
>
> See commit 10474ae8945ce ("KVM: Activate Virtualization On Demand").

Fine. This doesn't need to change ... until you load TDX. Once you
initialize the TDX module, no more out-of-tree VMMs for you.

That doesn't seem too insane. This is yet *ANOTHER* reason that doing
dynamic TDX module initialization is a good idea.

>> It's not wrong to say that TDX is a
>> KVM user. If KVm wants 'kvm_usage_count' to go back to 0, it can shut
>> down the TDX module. Then there's no PAMT to worry about.
>>
>> The shutdown would be something like:
>>
>> 1. TDX module shutdown
>> 2. Deallocate/Convert PAMT
>> 3. vmxoff
>>
>> Then, no SEAMCALL failure because of vmxoff can cause a PAMT-induced #MC
>> to be missed.
>
> The limitation is once the TDX module is shutdown, it cannot be initialized
> again unless it is runtimely updated.
>
> Long-termly, if we go this design then there might be other problems when other
> kernel components are using TDX. For example, the VT-d driver will need to be
> changed to support TDX-IO, and it will need to enable TDX module much earlier
> than KVM to do some initialization. It might need to some TDX work (e.g.,
> cleanup) while KVM is unloaded. I am not super familiar with TDX-IO but looks
> we might have some problem here if we go with such design.

The burden for who does vmxon will simply need to change from KVM itself
to some common code that KVM depends on. Probably not dissimilar to
those nutty (sorry folks, just calling it as I see 'em) multi-KVM module
patches that are floating around.