KVM: TDX: Implement callbacks for MSR operations

KVM: TDX: Implement callbacks for MSR operations

Add functions to implement MSR related callbacks, .set_msr(), .get_msr(),
and .has_emulated_msr(), for preparation of handling hypercalls from TDX
guest for PV RDMSR and WRMSR.  Ignore KVM_REQ_MSR_FILTER_CHANGED for TDX.

There are three classes of MSR virtualization for TDX.
- Non-configurable: TDX module directly virtualizes it. VMM can't configure
  it, the value set by KVM_SET_MSRS is ignored.
- Configurable: TDX module directly virtualizes it. VMM can configure it at
  VM creation time.  The value set by KVM_SET_MSRS is used.
- #VE case: TDX guest would issue TDG.VP.VMCALL<INSTRUCTION.{WRMSR,RDMSR}>
  and VMM handles the MSR hypercall. The value set by KVM_SET_MSRS is used.

For the MSRs belonging to the #VE case, the TDX module injects #VE to the
TDX guest upon RDMSR or WRMSR.  The exact list of such MSRs is defined in
TDX Module ABI Spec.

Upon #VE, the TDX guest may call TDG.VP.VMCALL<INSTRUCTION.{WRMSR,RDMSR}>,
which are defined in GHCI (Guest-Host Communication Interface) so that the
host VMM (e.g. KVM) can virtualize the MSRs.

TDX doesn't allow VMM to configure interception of MSR accesses.  Ignore
KVM_REQ_MSR_FILTER_CHANGED for TDX guest.  If the userspace has set any
MSR filters, it will be applied when handling
TDG.VP.VMCALL<INSTRUCTION.{WRMSR,RDMSR}> in a later patch.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Co-developed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20250227012021.1778144-9-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>