KVM/VMX: Avoid stack engine synchronization uop in __vmx_vcpu_run

KVM/VMX: Avoid stack engine synchronization uop in __vmx_vcpu_run

Avoid instructions with explicit uses of the stack pointer between
instructions that implicitly refer to it. The sequence of
POP %reg; ADD $x, %RSP; POP %reg forces emission of synchronization
uop to synchronize the value of the stack pointer in the stack engine
and the out-of-order core.

Using POP with the dummy register instead of ADD $x, %RSP results in a
smaller code size and faster code.

The patch also fixes the reference to the wrong register in the
nearby comment.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20220816211010.25693-1-ubizjak@gmail.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>