KVM: arm64: VHE: Save and restore host MDCR_EL2 value correctly

KVM: arm64: VHE: Save and restore host MDCR_EL2 value correctly

Prior to commit 75a5fbaf6623 ("KVM: arm64: Compute MDCR_EL2 at
vcpu_load()"), host MDCR_EL2 was saved correctly:

kvm_arch_vcpu_load()
  kvm_vcpu_load_debug() /* Doesn't touch hardware MDCR_EL2. */
  kvm_vcpu_load_vhe()
    __activate_traps_common()
       /* Saves host MDCR_EL2. */
       *host_data_ptr(host_debug_state.mdcr_el2) = read_sysreg(mdcr_el2)
       /* Writes VCPU MDCR_EL2. */
       write_sysreg(vcpu->arch.mdcr_el2, mdcr_el2)

The MDCR_EL2 value saved previously was restored in
kvm_arch_vcpu_put() -> kvm_vcpu_put_vhe().

After the aforementioned commit, host MDCR_EL2 is never saved:

kvm_arch_vcpu_load()
  kvm_vcpu_load_debug() /* Writes VCPU MDCR_EL2 */
  kvm_vcpu_load_vhe()
    __activate_traps_common()
       /* Saves **VCPU** MDCR_EL2. */
       *host_data_ptr(host_debug_state.mdcr_el2) = read_sysreg(mdcr_el2)
       /* Writes VCPU MDCR_EL2 a second time. */
       write_sysreg(vcpu->arch.mdcr_el2, mdcr_el2)

kvm_arch_vcpu_put() -> kvm_vcpu_put_vhe() then restores the VCPU MDCR_EL2
value. Also VCPU's MDCR_EL2 value gets written to hardware twice now.

Fix this by saving the host MDCR_EL2 in kvm_arch_vcpu_load() before it gets
overwritten by the VCPU's MDCR_EL2 value, and restore it on VCPU put.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250902130833.338216-3-alexandru.elisei@arm.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>