Use __kvm_mmu_new_pgd() via kvm_init_shadow_ept_mmu() to emulate
VMFUNC[EPTP_SWITCH] instead of nuking all MMUs.  EPTP_SWITCH is the EPT
equivalent of MOV to CR3, i.e. is a perfect fit for the common PGD flow,
the only hiccup being that A/D enabling is buried in the EPTP.  But, that
is easily handled by bouncing through kvm_init_shadow_ept_mmu().

Explicitly request a guest TLB flush if VPID is disabled.  Per Intel's
SDM, if VPID is disabled, "an EPTP-switching VMFUNC invalidates combined
mappings associated with VPID 0000H (for all PCIDs and for all EP4TA
values, where EP4TA is the value of bits 51:12 of EPTP)".

Note, this technically is a very bizarre bug fix of sorts if L2 is using
PAE paging, as avoiding the full MMU reload also avoids incorrectly
reloading the PDPTEs, which the SDM explicitly states are not touched:

  If PAE paging is in use, an EPTP-switching VMFUNC does not load the
  four page-directory-pointer-table entries (PDPTEs) from the
  guest-physical address in CR3. The logical processor continues to use
  the four guest-physical addresses already present in the PDPTEs. The
  guest-physical address in CR3 is not translated through the new EPT
  paging structures (until some operation that would load the PDPTEs).

In addition to optimizing L2's MMU shenanigans, avoiding the full reload
also optimizes L1's MMU as KVM_REQ_MMU_RELOAD wipes out all roots in both
root_mmu and guest_mmu.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210609234235.1244004-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>