Re: [PATCH v4 03/32] KVM: SVM: Flush the "current" TLB when activating AVIC

[Sean Christopherson]

On Thu, Dec 08, 2022, Maxim Levitsky wrote:
> On Wed, 2022-12-07 at 18:02 +0200, Maxim Levitsky wrote:
> On Sat, 2022-10-01 at 00:58 +0000, Sean Christopherson wrote:
> > --- a/arch/x86/kvm/svm/avic.c
> > +++ b/arch/x86/kvm/svm/avic.c
> > @@ -86,6 +86,12 @@ static void avic_activate_vmcb(struct vcpu_svm *svm)
> >                 /* Disabling MSR intercept for x2APIC registers */
> >                 svm_set_x2apic_msr_interception(svm, false);
> >         } else {
> > +               /*
> > +                * Flush the TLB, the guest may have inserted a non-APIC
> > +                * mapping into the TLB while AVIC was disabled.
> > +                */
> > +               kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, &svm->vcpu);
> > +
> >                 /* For xAVIC and hybrid-xAVIC modes */
> >                 vmcb->control.avic_physical_id |= AVIC_MAX_PHYSICAL_ID;
> >                 /* Enabling MSR intercept for x2APIC registers */
> 
> 
> I agree, that if guest disables APIC on a vCPU, this will lead to call to
> kvm_apic_update_apicv which will disable AVIC, but if other vCPUs don't
> disable it, the AVIC's private memslot will still be mapped and guest could
> read/write it from this vCPU, and its TLB mapping needs to be invalidated
> if/when APIC is re-enabled.
>  
> However I think that this adds an unnecessarily (at least in the future)
> performance penalty to AVIC nesting coexistence:
>  
> L1's AVIC is inhibited on each nested VM entry, and uninhibited on each
> nested VM exit, but while nested the guest can't really access it as it has
> its own NPT.
>  
> With this patch KVM will invalidate L1's TLB on each nested VM exit. KVM
> sadly already does this but this can be fixed (its another thing on my TODO
> list)
>  
> Note that APICv doesn't have this issue, it is not inhibited on nested VM
> entry/exit, thus this code is not performance sensitive for APICv.
>  
>  
> I somewhat vote again, as I said before to disable the APICv/AVIC memslot, if
> any of vCPUs have APICv/AVIC hardware disabled, because it is also more
> correct from an x86 perspective. I do wonder how often is the usage of having
> "extra" cpus but not using them, and thus having their APIC in disabled
> state.

There are legimate scenarios where a kernel might want to disable the APIC on
select CPUs, e.g. to offline SMT siblings in BIOS.  Whether or not doing that in
a VM makes sense is debatable, but we really have no way of knowing if there are
existing guests that selectively disable APICs.

> KVM does support adding new vCPUs on the fly, so this shouldn't be needed,
> and APICv inhibit in this case is just a perf regression.

Heh, "just" a perf regression.  Inhibiting APICv would definitely be a perf regression
that people care about, e.g. see the very recent bug fixes:

https://lore.kernel.org/all/20221116205123.18737-1-gedwards@xxxxxxx
https://lore.kernel.org/all/1669984574-32692-1-git-send-email-yuanzhaoxiong@xxxxxxxxx

Conceptually, I like the idea of inhibiting the APICv memslot if a vCPU has its
APIC hardware disabled.  But practically speaking, because KVM has allowed that
scenario for years, I don't think we should introduce such an inhibit and risk
regressing guests.

> Or at least do this only when APIC does back from hardware disabled state to
> enabled.

I have no objection to fine tuning this in follow-up, but for this bug fix I'd
much prefer to go with this minimal change.  The nested SVM TLB flushing issue
extends far beyond this one case, i.e. needs a non-trivial overhaul and an audit
of pretty every piece of SVM code that can interact with TLBs.