[PATCH V2] KVM: X86: fix tlb_flush_guest()

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <laijs@linux.alibaba.com>

For KVM_VCPU_FLUSH_TLB used in kvm_flush_tlb_multi(), the guest expects
the hypervisor do the operation that equals to native_flush_tlb_global()
or invpcid_flush_all() in the specified guest CPU.

When TDP is enabled, there is no problem to just flush the hardware
TLB of the specified guest CPU.

But when using shadowpaging, the hypervisor should have to sync the
shadow pagetable at first before flushing the hardware TLB so that
it can truely emulate the operation of invpcid_flush_all() in guest.

The problem exists since the first implementation of KVM_VCPU_FLUSH_TLB
in commit f38a7b75267f ("KVM: X86: support paravirtualized help for TLB
shootdowns").  But I don't think it would be a real world problem that
time since the local CPU's tlb is flushed at first in guest before queuing
KVM_VCPU_FLUSH_TLB to other CPUs.  It means that the hypervisor syncs the
shadow pagetable before seeing the corresponding KVM_VCPU_FLUSH_TLBs.

After commit 4ce94eabac16 ("x86/mm/tlb: Flush remote and local TLBs
concurrently"), the guest doesn't flush local CPU's tlb at first and
the hypervisor can handle other VCPU's KVM_VCPU_FLUSH_TLB earlier than
local VCPU's tlb flush and might flush the hardware tlb without syncing
the shadow pagetable beforehand.

Cc: Maxim Levitsky <mlevitsk@redhat.com>
Fixes: f38a7b75267f ("KVM: X86: support paravirtualized help for TLB shootdowns")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
Changed from V1
	Use kvm_mmu_unload() instead of KVM_REQ_MMU_RELOAD to avoid
	causing unneeded iteration of vcpu_enter_guest().

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index bbc4e04e67ad..27248e330767 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3072,6 +3072,22 @@ static void kvm_vcpu_flush_tlb_all(struct kvm_vcpu *vcpu)
 static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu)
 {
 	++vcpu->stat.tlb_flush;
+
+	if (!tdp_enabled) {
+		/*
+		 * When two dimensional paging is not enabled, the
+		 * operation should equal to native_flush_tlb_global()
+		 * or invpcid_flush_all() on the guest's behalf via
+		 * synchronzing shadow pagetable and flushing.
+		 *
+		 * kvm_mmu_unload() results consequent kvm_mmu_load()
+		 * before entering guest which will do the required
+		 * pagetable synchronzing and TLB flushing.
+		 */
+		kvm_mmu_unload(vcpu);
+		return;
+	}
+
 	static_call(kvm_x86_tlb_flush_guest)(vcpu);
 }
 
-- 
2.19.1.6.gb485710b