KVM: x86: Add support for RDMSR/WRMSRNS w/ immediate on Intel

Add support for the immediate forms of RDMSR and WRMSRNS (currently
Intel-only).  The immediate variants are only valid in 64-bit mode, and
use a single general purpose register for the data (the register is also
encoded in the instruction, i.e. not implicit like regular RDMSR/WRMSR).

The immediate variants are primarily motivated by performance, not code
size: by having the MSR index in an immediate, it is available *much*
earlier in the CPU pipeline, which allows hardware much more leeway about
how a particular MSR is handled.

Intel VMX support for the immediate forms of MSR accesses communicates
exit information to the host as follows:

  1) The immediate form of RDMSR uses VM-Exit Reason 84.

  2) The immediate form of WRMSRNS uses VM-Exit Reason 85.

  3) For both VM-Exit reasons 84 and 85, the Exit Qualification field is
     set to the MSR index that triggered the VM-Exit.

  4) Bits 3 ~ 6 of the VM-Exit Instruction Information field are set to
     the register encoding used by the immediate form of the instruction,
     i.e. the destination register for RDMSR, and the source for WRMSRNS.

  5) The VM-Exit Instruction Length field records the size of the
     immediate form of the MSR instruction.

To deal with userspace RDMSR exits, stash the destination register in a
new kvm_vcpu_arch field, similar to cui_linear_rip, pio, etc.
Alternatively, the register could be saved in kvm_run.msr or re-retrieved
from the VMCS, but the former would require sanitizing the value to ensure
userspace doesn't clobber the value to an out-of-bounds index, and the
latter would require a new one-off kvm_x86_ops hook.

Don't bother adding support for the instructions in KVM's emulator, as the
only way for RDMSR/WRMSR to be encountered is if KVM is emulating large
swaths of code due to invalid guest state, and a vCPU cannot have invalid
guest state while in 64-bit mode.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
[sean: minor tweaks, massage and expand changelog]
Link: https://lore.kernel.org/r/20250805202224.1475590-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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