linux-kernel - Re: [PATCH] KVM: SVM: Disable TDP MMU when running on Hyper-V

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Date:   Wed, 8 Mar 2023 18:22:42 +0100
From:   Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Tianyu Lan <ltykernel@...il.com>,
        Michael Kelley <mikelley@...rosoft.com>
Subject: Re: [PATCH] KVM: SVM: Disable TDP MMU when running on Hyper-V

On 08/03/2023 16:55, Jeremi Piotrowski wrote:
> 
> 
> On 08/03/2023 01:39, Sean Christopherson wrote:
>> On Wed, Mar 08, 2023, Paolo Bonzini wrote:
>>> On Tue, Mar 7, 2023 at 6:36 PM Sean Christopherson <seanjc@...gle.com> wrote:
>>>> Thinking about this more, I would rather revert commit 1e0c7d40758b ("KVM: SVM:
>>>> hyper-v: Remote TLB flush for SVM") or fix the thing properly straitaway.  KVM
>>>> doesn't magically handle the flushes correctly for the shadow/legacy MMU, KVM just
>>>> happens to get lucky and not run afoul of the underlying bugs.
>>>
>>> I don't think it's about luck---the legacy MMU's zapping/invalidation
>>> seems to invoke the flush hypercall correctly:
>>
>> ...for the paths that Jeremi has exercised, and for which a stale TLB entry is
>> fatal to L2.  E.g. kvm_unmap_gfn_range() does not have a range-based TLB flush
>> in its path and fully relies on the buggy kvm_flush_remote_tlbs().
>>
> 
> Why do you say "buggy kvm_flush_remote_tlbs"? kvm_flush_remote_tlbs calls the hypercall
> that is needed, I don't see how this might be an issue of a missing "range-based TLB flush".
> 
> kvm_unmap_gfn_range is called from kvm_mmu_notifier_invalidate_range_start and 'flush_on_ret=true'
> is set, so it is followed by kvm_flush_remote_tlbs which calls hv_remote_flush_tlb.
> 
>> In other words, KVM is getting lucky :-)
>>
>>> Jeremi, did you ever track the call stack where
>>> hyperv_nested_flush_guest_mapping is triggered?
>>
>> I don't think it matters.  As above, it only takes one path where KVM is fully
>> relying on kvm_flush_remote_tlbs() for the whole thing to fall apart

Slowly I'm starting to understand what we've been talking about, thank you :)

Paolo/Sean, what do you think about smth like the following, except I would make
it SVM only, and I'd need to think about what to do with the return.
I believe this accurately reflects what the optimization is about. hv_track_root_tdp
is called from kvm_mmu_load_pgd, which covers both kvm_mmu_load and kvm_mmu_new_pgd
(which requests KVM_REQ_LOAD_MMU_PGD).

diff --git a/arch/x86/kvm/kvm_onhyperv.c b/arch/x86/kvm/kvm_onhyperv.c
index 482d6639ef88..6a5bd3cbace8 100644
--- a/arch/x86/kvm/kvm_onhyperv.c
+++ b/arch/x86/kvm/kvm_onhyperv.c
@@ -29,6 +29,18 @@ static inline int hv_remote_flush_root_tdp(hpa_t root_tdp,
 		return hyperv_flush_guest_mapping(root_tdp);
 }
 
+static int hv_vcpu_flush_tlb_current(struct kvm_vcpu *vcpu)
+{
+	struct kvm_arch *kvm_arch = &vcpu->kvm->arch;
+	hpa_t root_tdp = vcpu->arch.hv_root_tdp;
+	int ret;
+
+	ret = hyperv_flush_guest_mapping(root_tdp);
+	if (!ret)
+		kvm_arch->hv_root_tdp = root_tdp;
+	return ret;
+}
+
 int hv_remote_flush_tlb_with_range(struct kvm *kvm,
 		struct kvm_tlb_range *range)
 {
@@ -101,8 +113,10 @@ void hv_track_root_tdp(struct kvm_vcpu *vcpu, hpa_t root_tdp)
 	if (kvm_x86_ops.tlb_remote_flush == hv_remote_flush_tlb) {
 		spin_lock(&kvm_arch->hv_root_tdp_lock);
 		vcpu->arch.hv_root_tdp = root_tdp;
-		if (root_tdp != kvm_arch->hv_root_tdp)
+		if (root_tdp != kvm_arch->hv_root_tdp) {
 			kvm_arch->hv_root_tdp = INVALID_PAGE;
+			hv_vcpu_flush_tlb_current(vcpu);
+		}
 		spin_unlock(&kvm_arch->hv_root_tdp_lock);
 	}
 }