KVM: TDX: Enable guest access to LMCE related MSRs

KVM: TDX: Enable guest access to LMCE related MSRs

Allow TDX guest to configure LMCE (Local Machine Check Event) by handling
MSR IA32_FEAT_CTL and IA32_MCG_EXT_CTL.

MCE and MCA are advertised via cpuid based on the TDX module spec.  Guest
kernel can access IA32_FEAT_CTL to check whether LMCE is opted-in by the
platform or not.  If LMCE is opted-in by the platform, guest kernel can
access IA32_MCG_EXT_CTL to enable/disable LMCE.

Handle MSR IA32_FEAT_CTL and IA32_MCG_EXT_CTL for TDX guests to avoid
failure when a guest accesses them with TDG.VP.VMCALL<MSR> on #VE.  E.g.,
Linux guest will treat the failure as a #GP(0).

Userspace VMM may not opt-in LMCE by default, e.g., QEMU disables it by
default, "-cpu lmce=on" is needed in QEMU command line to opt-in it.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
[binbin: rework changelog]
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Message-ID: <20250227012021.1778144-11-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>