KVM: TDX: Handle vCPU dissociation

Handle vCPUs dissociations by invoking SEAMCALL TDH.VP.FLUSH which flushes
the address translation caches and cached TD VMCS of a TD vCPU in its
associated pCPU.

In TDX, a vCPUs can only be associated with one pCPU at a time, which is
done by invoking SEAMCALL TDH.VP.ENTER. For a successful association, the
vCPU must be dissociated from its previous associated pCPU.

To facilitate vCPU dissociation, introduce a per-pCPU list
associated_tdvcpus. Add a vCPU into this list when it's loaded into a new
pCPU (i.e. when a vCPU is loaded for the first time or migrated to a new
pCPU).

vCPU dissociations can happen under below conditions:
- On the op hardware_disable is called.
  This op is called when virtualization is disabled on a given pCPU, e.g.
  when hot-unplug a pCPU or machine shutdown/suspend.
  In this case, dissociate all vCPUs from the pCPU by iterating its
  per-pCPU list associated_tdvcpus.

- On vCPU migration to a new pCPU.
  Before adding a vCPU into associated_tdvcpus list of the new pCPU,
  dissociation from its old pCPU is required, which is performed by issuing
  an IPI and executing SEAMCALL TDH.VP.FLUSH on the old pCPU.
  On a successful dissociation, the vCPU will be removed from the
  associated_tdvcpus list of its previously associated pCPU.

- On tdx_mmu_release_hkid() is called.
  TDX mandates that all vCPUs must be disassociated prior to the release of
  an hkid. Therefore, dissociation of all vCPUs is a must before executing
  the SEAMCALL TDH.MNG.VPFLUSHDONE and subsequently freeing the hkid.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Co-developed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Message-ID: <20241112073858.22312-1-yan.y.zhao@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>