x86/tdx: Disable unnecessary virtualization exceptions

Originally, #VE was defined as the TDX behavior in order to support
paravirtualization of x86 features that can’t be virtualized by the TDX
module. The intention is that if guest software wishes to use such a
feature, it implements some logic to support this. This logic resides in
the #VE exception handler it may work in cooperation with the host VMM.

Theoretically, the guest TD’s #VE handler was supposed to act as a "TDX
enlightenment agent" inside the TD. However, in practice, the #VE
handler is simplistic:

  - #VE on CPUID is handled by returning all-0 to the code which
    executed CPUID. In many cases, an all-0 value is not the correct
    value, and may cause improper operation.

  - #VE on RDMSR is handled by requesting the MSR value from the host
    VMM. This is prone to security issues since the host VMM is
    untrusted. It may also be functionally incorrect in case the
    expected operation is to paravirtualize some CPU functionality.

Newer TDX modules provide a "REDUCE_VE" feature. When enabled, it
drastically cuts cases when guests receive #VE on MSR and CPUID
accesses. Basically, instead of punting the problem to the VMM, the
TDX module fills in good data. What the TDX module provides is
obviously highly specific to the MSR or CPUID. This is all spelled
out in excruciating detail in the TDX specs.

Enable REDUCE_VE. Make TDX guest behaviour less odd, and closer to
how a normal CPU behaves.

Note that enabling of the feature doesn't eliminate need in #VE handler
for CPUID and MSR accesses. Some MSRs still generate #VE (notably
APIC-related) and kernel needs CPUID #VE handler to ask VMM for leafs in
hypervisor range.

[ dhansen: changelog tweaks, rename/rework VE reduction function ]

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/all/20241202072431.447380-1-kirill.shutemov%40linux.intel.com

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