KVM: x86/mmu: Set mmio_value to '0' if reserved #PF can't be generated

mainline inclusion
from mainline-v5.8-rc1
commit 6129ed87
category: feature
bugzilla:
CVE: NA

--------------------------------

Set the mmio_value to '0' instead of simply clearing the present bit to
squash a benign warning in kvm_mmu_set_mmio_spte_mask() that complains
about the mmio_value overlapping the lower GFN mask on systems with 52
bits of PA space.

Opportunistically clean up the code and comments.

Cc: stable@vger.kernel.org
Fixes: d43e2675 ("KVM: x86: only do L1TF workaround on affected processors")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200527084909.23492-1-sean.j.christopherson@intel.com>
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn> #openEuler_contributor
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>