Re: [PATCH v2 07/10] KVM: x86: Always populate L1 GPA for KVM_MAP_MEMORY

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

I wouldn't call myself much of an expert on nested, but...

On Wed, 2024-04-10 at 15:07 -0700, isaku.yamahata@intel.com wrote:
> There are several options to populate L1 GPA irrelevant to vCPU mode.
> - Switch vCPU MMU only: This patch.
>   Pros: Concise implementation.
>   Cons: Heavily dependent on the KVM MMU implementation.

Is switching just the MMU enough here? Won't the MTRRs and other vcpu bits be
wrong?

> - Use kvm_x86_nested_ops.get/set_state() to switch to/from guest mode.
>   Use __get/set_sregs2() to switch to/from SMM mode.
>   Pros: straightforward.
>   Cons: This may cause unintended side effects.

Cons make sense.

> - Refactor KVM page fault handler not to pass vCPU. Pass around necessary
>   parameters and struct kvm.
>   Pros: The end result will have clearly no side effects.
>   Cons: This will require big refactoring.

But doesn't the fault handler need the vCPU state?

> - Return error on guest mode or SMM mode:  Without this patch.
>   Pros: No additional patch.
>   Cons: Difficult to use.

Hmm... For the non-TDX use cases this is just an optimization, right? For TDX
there shouldn't be an issue. If so, maybe this last one is not so horrible.