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

[Sean Christopherson <seanjc@google.com>]

+Maxim - A proper fix for this bug might fix your shadow paging + win10 boot
         issue, this also affects the KVM_REQ_HV_TLB_FLUSH used for HyperV PV
	 flushing.

On Thu, May 27, 2021, Paolo Bonzini wrote:
> On 27/05/21 04:39, Lai Jiangshan wrote:
> > 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.
> 
> Can you explain why?

KVM's unsync logic hinges on guest TLB flushes.  For page permission modifications
that require a TLB flush to take effect, e.g. making a writable page read-only,
KVM waits until the guest explicitly does said flush to propagate the changes to
the shadow page tables.  E.g. failure to sync PTEs could result in a read-only 4k
page being writable when the guest expects it to be read-only.

> Also it is simpler to handle this in kvm_vcpu_flush_tlb_guest, using "if
> (tdp_enabled).  This provides also a single, good place to add a comment
> with the explanation of what invalid entries KVM_REQ_RELOAD is presenting.

Ya.  

KVM_REQ_MMU_RELOAD is overkill, nuking the shadow page tables will completely
offset the performance gains of the paravirtualized flush.

And making a request won't work without revamping the order of request handling
in vcpu_enter_guest(), e.g. KVM_REQ_MMU_RELOAD and KVM_REQ_MMU_SYNC are both
serviced before KVM_REQ_STEAL_UPDATE.

Cleaning up and documenting the MMU related requests is on my todo list, but the
immediate fix should be tiny and I can do my cleanups on top.

I believe the minimal fix is:

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 81ab3b8f22e5..b0072063f9bf 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3072,6 +3072,9 @@ 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)
+               kvm_mmu_sync_roots(vcpu);
        static_call(kvm_x86_tlb_flush_guest)(vcpu);
 }