Re: "KVM: x86/mmu: Overhaul TDP MMU zapping and flushing" breaks SVM on Hyper-V

Sean Christopherson
2023-02-13T19:12:04Z

On Mon, Feb 13, 2023, Paolo Bonzini wrote:
> On 2/13/23 18:38, Sean Christopherson wrote:
> > On Fri, Feb 10, 2023, Jeremi Piotrowski wrote:
> > > Hi Paolo/Sean,
> > >
> > > We've noticed that changes introduced in "KVM: x86/mmu: Overhaul TDP MMU
> > > zapping and flushing" conflict with a nested Hyper-V enlightenment that is
> > > always enabled on AMD CPUs (HV_X64_NESTED_ENLIGHTENED_TLB). The scenario that
> > > is affected is L0 Hyper-V + L1 KVM on AMD,
> >
> > Do you see issues with Intel and HV_X64_NESTED_GUEST_MAPPING_FLUSH? IIUC, on the
> > KVM side, that setup is equivalent to HV_X64_NESTED_ENLIGHTENED_TLB.
>
> My reading of the spec[1] is that HV_X64_NESTED_ENLIGHTENED_TLB will cause
> svm_flush_tlb_current to behave (in Intel parlance) as an INVVPID rather
> than an INVEPT.

Oh! Good catch! Yeah, that'll be a problem. Copy-pasting the relevant snippet
so future me doesn't have to reread the spec:

If the nested hypervisor opts into the enlightenment, ASID invalidations just
flush TLB entires derived from first level address translation (i.e. the
virtual address space).

Specifically, the "missing" flushes when a root's (nCR3) refcount goes to zero
are expected because KVM relies on flushing via svm_flush_tlb_current() when the
old, stale root might be reused. That would lead to consuming stale entries when
reusing a previously freed root.

int kvm_mmu_load(struct kvm_vcpu *vcpu)
{
int r;

...

/*
* Flush any TLB entries for the new root, the provenance of the root
* is unknown. Even if KVM ensures there are no stale TLB entries
* for a freed root, in theory another hypervisor could have left
* stale entries. Flushing on alloc also allows KVM to skip the TLB
* flush when freeing a root (see kvm_tdp_mmu_put_root()).
*/
static_call(kvm_x86_flush_tlb_current)(vcpu);
out:
return r;
}

> So svm_flush_tlb_current has to be changed to also add a
> call to HvCallFlushGuestPhysicalAddressSpace. I'm not sure if that's a good
> idea though.

That's not strictly necessary, e.g. flushes from kvm_invalidate_pcid() and
kvm_post_set_cr4() don't need to effect a full flush. I believe the virtual
address flush is also sufficient for avic_activate_vmcb(). Nested (from KVM's
perspective, i.e. running L3) can just be mutually exclusive with
HV_X64_NESTED_ENLIGHTENED_TLB.

That just leaves kvm_mmu_new_pgd()'s force_flush_and_sync_on_reuse and the
aforementioned kvm_mmu_load().

That said, the above cases where a virtual address flush is sufficient are
rare operations when using NPT, so adding a new KVM_REQ_TLB_FLUSH_ROOT or
whatever probably isn't worth doing.

> First, that's a TLB shootdown rather than just a local thing;
> flush_tlb_current is supposed to be relatively cheap, and there would be a
> lot of them because of the unconditional calls to
> nested_svm_transition_tlb_flush on vmentry/vmexit.

This isn't a nested scenario for KVM though.

> Second, while the nCR3 matches across virtual processors for SVM, the (nCR3,
> ASID) pair does not, so it doesn't even make much sense to do a TLB
> shootdown.
>
> Depending on the performance results of adding the hypercall to
> svm_flush_tlb_current, the fix could indeed be to just disable usage of
> HV_X64_NESTED_ENLIGHTENED_TLB.

Minus making nested SVM (L3) mutually exclusive, I believe this will do the trick:

---
arch/x86/kvm/kvm_onhyperv.c | 9 +++++++++
arch/x86/kvm/kvm_onhyperv.h | 4 ++++
arch/x86/kvm/svm/svm.c | 3 +++
3 files changed, 16 insertions(+)

diff --git a/arch/x86/kvm/kvm_onhyperv.c b/arch/x86/kvm/kvm_onhyperv.c
index 482d6639ef88..e03e9296c1cf 100644
--- a/arch/x86/kvm/kvm_onhyperv.c
+++ b/arch/x86/kvm/kvm_onhyperv.c
@@ -107,3 +107,12 @@ void hv_track_root_tdp(struct kvm_vcpu *vcpu, hpa_t root_tdp)
}
}
EXPORT_SYMBOL_GPL(hv_track_root_tdp);
+
+void hv_flush_tlb_current(struct kvm_vcpu *vcpu)
+{
+ if (kvm_x86_ops.tlb_remote_flush != hv_remote_flush_tlb)
+ return;
+
+ WARN_ON_ONCE(hyperv_flush_guest_mapping(vcpu->arch.mmu->root.hpa));
+}
+EXPORT_SYMBOL_GPL(hv_flush_tlb_current);
diff --git a/arch/x86/kvm/kvm_onhyperv.h b/arch/x86/kvm/kvm_onhyperv.h
index 287e98ef9df3..30789dfd3544 100644
--- a/arch/x86/kvm/kvm_onhyperv.h
+++ b/arch/x86/kvm/kvm_onhyperv.h
@@ -11,10 +11,14 @@ int hv_remote_flush_tlb_with_range(struct kvm *kvm,
struct kvm_tlb_range *range);
int hv_remote_flush_tlb(struct kvm *kvm);
void hv_track_root_tdp(struct kvm_vcpu *vcpu, hpa_t root_tdp);
+void hv_flush_tlb_current(struct kvm_vcpu *vcpu);
#else /* !CONFIG_HYPERV */
static inline void hv_track_root_tdp(struct kvm_vcpu *vcpu, hpa_t root_tdp)
{
}
+static inline void hv_flush_tlb_current(struct kvm_vcpu *vcpu)
+{
+}
#endif /* !CONFIG_HYPERV */

#endif
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index b43775490074..bfc71dfa8482 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3733,6 +3733,9 @@ static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
{
struct vcpu_svm *svm = to_svm(vcpu);

+ /* blah blah blah */
+ hv_flush_tlb_current(vcpu);
+
/*
* Unlike VMX, SVM doesn't provide a way to flush only NPT TLB entries.
* A TLB flush for the current ASID flushes both "host" and "guest" TLB

base-commit: 9fa259abdb42051e5ab4cbf0bc0cd21adcf95a4f
--