Re: [PATCH v10 02/39] KVM: x86: hyper-v: Resurrect dedicated KVM_REQ_HV_TLB_FLUSH flag

Vitaly Kuznetsov
2022-09-22T15:37:16Z

Sean Christopherson <seanjc@google.com> writes:

> On Thu, Sep 22, 2022, Vitaly Kuznetsov wrote:
>> Now let's get to VMX and the point of my confusion (and thanks in
>> advance for educating me!):
>> AFAIU, when EPT is in use:
>> KVM_REQ_TLB_FLUSH_CURRENT == invept
>> KVM_REQ_TLB_FLUSH_GUEST = invvpid
>>
>> For "normal" mappings (which are mapped on both stages) this is the same
>> thing as they're 'tagged' with both VPID and 'EPT root'. The question is
>> what's left. Given your comment, do I understand correctly that in case
>> of an invalid mapping in the guest (GVA doesn't resolve to a GPA), this
>> will only be tagged with VPID but not with 'EPT root' (as the CPU never
>> reached to the second translation stage)? We certainly can't ignore
>> these. Another (probably pure theoretical question) is what are the
>> mappings which are tagged with 'EPT root' but don't have a VPID tag?
>
> Intel puts mappings into three categories, which for non-root mode equates to:
>
> linear == GVA => GPA
> guest-physical == GPA => HPA
> combined == GVA => HPA
>
> and essentially the categories that consume the GVA are tagged with the VPID
> (linear and combined), and categories that consume the GPA are tagged with the
> EPTP address (guest-physical and combined).
>
>> Are these the mapping which happen when e.g. vCPU has paging disabled?
>
> No, these mappings can be created at all times. Even with CR0.PG=1, the guest
> can generate GPAs without going through a GVA=>GPA translation, e.g. the page tables
> themselves, RTIT (Intel PT) addresses, etc... And even for combined/full
> translations, the CPU can insert TLB entries for just the GPA=>HPA part.
>
> E.g. when a page is allocated by/for userspace, the kernel will zero the page using
> the kernel's direct map, but userspace will access the page via a different GVA.
> I.e. the guest effectively aliases GPA(x) with GVA(k) and GVA(u). By inserting
> the GPA(x) => HPA(y) into the TLB, when guest userspace access GVA(u), the CPU
> encounters a TLB miss on GVA(u) => GPA(x), but gets a TLB hit on GPA(x) => HPA(y).
>
> Separating EPT flushes from VPID (and PCID) flushes allows the CPU to retain
> the partial TLB entries, e.g. a host change in the EPT tables will result in the
> guest-physical and combined mappings being invalidated, but linear mappings can
> be kept.
>

Thanks a bunch! For some reason I though it's always the full thing (combined)
which is tagged with both VPID/PCID and EPTP and linear/guest-physical
are just 'corner' cases (but are still combined and tagged). Apparently,
it's not like that.

> I'm 99% certain AMD also caches partial entries, e.g. see the blurb on INVLPGA
> not affecting NPT translations, AMD just doesn't provide a way for the host to
> flush _only_ NPT translations. Maybe the performance benefits weren't significant
> enough to justify the extra complexity?
>
>> These are probably unrelated to Hyper-V TLB flushing.
>>
>> To preserve the 'small' optimization, we can probably move
>> kvm_clear_request(KVM_REQ_HV_TLB_FLUSH, vcpu);
>>
>> to nested_svm_transition_tlb_flush() or, in case this sounds too
>> hackish
>
> Move it to svm_flush_tlb_current(), because the justification is that on SVM,
> flushing "current" TLB entries also flushes "guest" TLB entries due to the more
> coarse-grained ASID-based TLB flush. E.g.
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index dd599afc85f5..a86b41503723 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3737,6 +3737,13 @@ static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
> {
> struct vcpu_svm *svm = to_svm(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
> + * entries, and thus is a superset of Hyper-V's fine grained flushing.
> + */
> + kvm_hv_vcpu_purge_flush_tlb(vcpu);
> +
> /*
> * Flush only the current ASID even if the TLB flush was invoked via
> * kvm_flush_remote_tlbs(). Although flushing remote TLBs requires all
>
>> we can drop it for now and add it to the (already overfull)
>> bucket of the "optimize nested_svm_transition_tlb_flush()".
>
> I think even long term, purging Hyper-V's FIFO in svm_flush_tlb_current() is the
> correct/desired behavior. This doesn't really have anything to do with nSVM,
> it's all about SVM not providing a way to flush only NPT entries.

True that, silly me forgot that even without any nesting, Hyper-V TLB
flush after svm_flush_tlb_current() makes no sense.

>

--
Vitaly