Re: [PATCH] KVM: SVM: Flush Hyper-V TLB when required

Vitaly Kuznetsov
2023-03-22T16:54:38Z

Sean Christopherson <seanjc@google.com> writes:

> On Mon, Mar 20, 2023, Jeremi Piotrowski wrote:
>> ---
>> arch/x86/kvm/kvm_onhyperv.c | 23 +++++++++++++++++++++++
>> arch/x86/kvm/kvm_onhyperv.h | 5 +++++
>> arch/x86/kvm/svm/svm.c | 18 +++++++++++++++---
>> 3 files changed, 43 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/kvm_onhyperv.c b/arch/x86/kvm/kvm_onhyperv.c
>> index 482d6639ef88..036e04c0a161 100644
>> --- a/arch/x86/kvm/kvm_onhyperv.c
>> +++ b/arch/x86/kvm/kvm_onhyperv.c
>> @@ -94,6 +94,29 @@ int hv_remote_flush_tlb(struct kvm *kvm)
>> }
>> EXPORT_SYMBOL_GPL(hv_remote_flush_tlb);
>>
>> +void hv_flush_tlb_current(struct kvm_vcpu *vcpu)
>> +{
>> + struct kvm_arch *kvm_arch = &vcpu->kvm->arch;
>> + hpa_t root_tdp = vcpu->arch.mmu->root.hpa;
>> +
>> + if (kvm_x86_ops.tlb_remote_flush == hv_remote_flush_tlb && VALID_PAGE(root_tdp)) {
>> + spin_lock(&kvm_arch->hv_root_tdp_lock);
>> + if (kvm_arch->hv_root_tdp != root_tdp) {
>> + hyperv_flush_guest_mapping(root_tdp);
>> + kvm_arch->hv_root_tdp = root_tdp;
>
> In a vacuum, accessing kvm_arch->hv_root_tdp in the flush path is wrong. This
> likely fixes the issues you are seeing because the KVM bug only affects the case
> when KVM is loading a new root (that used to be valid), in which case hv_root_tdp
> is guaranteed to be different. But KVM should not rely on that behavior, i.e. if
> KVM says flush, then we flush. There might be scenarios where the flush is
> unnecessary, but those flushes should be elided by the code that knows the flush
> is unnecessary, not in this common code just because the target root is the
> globally shared root.
>
> Somewhat of a moot point, but setting hv_root_tdp to root_tdp is also wrong. KVM's
> behavior is that hv_root_tdp points at a valid root if and only if all vCPUs share
> said root. E.g. invoking this when vCPUs have different roots will "corrupt"
> hv_root_tdp and possibly cause a remote flush to do the wrong thing.
>
>> + }
>> + spin_unlock(&kvm_arch->hv_root_tdp_lock);
>> + }
>> +}
>> +EXPORT_SYMBOL_GPL(hv_flush_tlb_current);
>> +
>> +void hv_flush_tlb_all(struct kvm_vcpu *vcpu)
>> +{
>> + if (WARN_ON_ONCE(kvm_x86_ops.tlb_remote_flush == hv_remote_flush_tlb))
>
> Hmm, looking at the KVM code, AFAICT KVM only enables enlightened_npt_tlb for L1
> (L1 from KVM's perspective) as svm_hv_init_vmcb() is only ever called with vmcb01,
> never with vmcb02. I don't know if that's intentional, but I do think it means
> KVM can skip the Hyper-V flush for vmcb02 and instead rely on the ASID flush,
> i.e. KVM can do the Hyper-V iff enlightened_npt_tlb is set in the current VMCB.
> And that should continue to work if KVM does ever enabled enlightened_npt_tlb for L2.
>
>> + hv_remote_flush_tlb(vcpu->kvm);
>> +}
>> +EXPORT_SYMBOL_GPL(hv_flush_tlb_all);
>
> I'd rather not add helpers to the common KVM code. I do like minimizing the amount
> of #ifdeffery, but defining these as common helpers makes it seem like VMX-on-HyperV
> is broken, i.e. raises the question of why VMX doesn't use these helpers when running
> on Hyper-V.
>
> I'm thinking this?
>
> ---
> arch/x86/kvm/svm/svm.c | 39 ++++++++++++++++++++++++++++++---
> arch/x86/kvm/svm/svm_onhyperv.h | 7 ++++++
> 2 files changed, 43 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 70183d2271b5..ab97fe8f1d81 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3746,7 +3746,7 @@ static void svm_enable_nmi_window(struct kvm_vcpu *vcpu)
> svm->vmcb->save.rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
> }
>
> -static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
> +static void svm_flush_tlb_asid(struct kvm_vcpu *vcpu)
> {
> struct vcpu_svm *svm = to_svm(vcpu);
>
> @@ -3770,6 +3770,39 @@ static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
> svm->current_vmcb->asid_generation--;
> }
>
> +static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
> +{
> +#if IS_ENABLED(CONFIG_HYPERV)
> + hpa_t root_tdp = vcpu->arch.mmu->root.hpa;
> +
> + /*
> + * When running on Hyper-V with EnlightenedNptTlb enabled, explicitly
> + * flush the NPT mappings via hypercall as flushing the ASID only
> + * affects virtual to physical mappings, it does not invalidate guest
> + * physical to host physical mappings.
> + */
> + if (svm_hv_is_enlightened_tlb_enabled(vcpu) && VALID_PAGE(root_tdp))
> + hyperv_flush_guest_mapping(root_tdp);
> +#endif
> + svm_flush_tlb_asid(vcpu);
> +}
> +
> +static void svm_flush_tlb_all(struct kvm_vcpu *vcpu)
> +{
> +#if IS_ENABLED(CONFIG_HYPERV)
> + /*
> + * When running on Hyper-V with EnlightenedNptTlb enabled, remote TLB
> + * flushes should be routed to hv_remote_flush_tlb() without requesting
> + * a "regular" remote flush. Reaching this point means either there's
> + * a KVM bug or a prior hv_remote_flush_tlb() call failed, both of
> + * which might be fatal to the the guest. Yell, but try to recover.
> + */
> + if (WARN_ON_ONCE(svm_hv_is_enlightened_tlb_enabled(vcpu)))
> + hv_remote_flush_tlb(vcpu->kvm);
> +#endif
> + svm_flush_tlb_asid(vcpu);
> +}
> +
> static void svm_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t gva)
> {
> struct vcpu_svm *svm = to_svm(vcpu);
> @@ -4762,10 +4795,10 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
> .set_rflags = svm_set_rflags,
> .get_if_flag = svm_get_if_flag,
>
> - .flush_tlb_all = svm_flush_tlb_current,
> + .flush_tlb_all = svm_flush_tlb_all,
> .flush_tlb_current = svm_flush_tlb_current,
> .flush_tlb_gva = svm_flush_tlb_gva,
> - .flush_tlb_guest = svm_flush_tlb_current,
> + .flush_tlb_guest = svm_flush_tlb_asid,
>
> .vcpu_pre_run = svm_vcpu_pre_run,
> .vcpu_run = svm_vcpu_run,
> diff --git a/arch/x86/kvm/svm/svm_onhyperv.h b/arch/x86/kvm/svm/svm_onhyperv.h
> index cff838f15db5..d91e019fb7da 100644
> --- a/arch/x86/kvm/svm/svm_onhyperv.h
> +++ b/arch/x86/kvm/svm/svm_onhyperv.h
> @@ -15,6 +15,13 @@ static struct kvm_x86_ops svm_x86_ops;
>
> int svm_hv_enable_l2_tlb_flush(struct kvm_vcpu *vcpu);
>
> +static inline bool svm_hv_is_enlightened_tlb_enabled(struct kvm_vcpu *vcpu)
> +{
> + struct hv_vmcb_enlightenments *hve = &to_svm(vcpu)->vmcb->control.hv_enlightenments;
> +
> + return !!hve->hv_enlightenments_control.enlightened_npt_tlb;

In theory, we should not look at Hyper-V enlightenments in VMCB control
just because our kernel has CONFIG_HYPERV enabled. I'd suggest we add a
real check that we're running on Hyper-V and we can do it the same way
it is done in svm_hv_hardware_setup()/svm_hv_init_vmcb():

return (ms_hyperv.nested_features & HV_X64_NESTED_ENLIGHTENED_TLB)
&& !!hve->hv_enlightenments_control.enlightened_npt_tlb;

(untested).

> +}
> +
> static inline void svm_hv_init_vmcb(struct vmcb *vmcb)
> {
> struct hv_vmcb_enlightenments *hve = &vmcb->control.hv_enlightenments;
>
> base-commit: 50f13998451effea5c5fdc70fe576f8b435d6224

--
Vitaly