Re: [PATCH] KVM: SVM: make svm_flush_tlb_gva do a full asid flush if NPT enabled

[Yosry Ahmed]

> I also love Sean's idea, I think it's good to harden against this by
> nullifying flush_tlb_gva, and maybe add a helper that does the
> fallback:
>
> static void kvm_vcpu_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t gva)
> {
>        if (kvm_x86_ops.flush_tlb_gva)
>               kvm_x86_call(flush_tlb_gva)(vcpu, addr);
>        else
>               kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu);
> }
>
> Hmm actually we check KVM_REQ_TLB_FLUSH_GUEST before
> KVM_REQ_HV_TLB_FLUSH, so maybe just call kvm_vcpu_flush_tlb_guest()
> directly for the fallback:
>
> static void kvm_vcpu_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t gva)
> {
>        if (kvm_x86_ops.flush_tlb_gva)
>               kvm_x86_call(flush_tlb_gva)(vcpu, addr);
>        else
>               kvm_vcpu_flush_tlb_guest(vcpu);
> }
>
> And if we go this route, I think we can key off the presence of
> flush_tlb_gva in kvm_hv_vcpu_flush_tlb() instead of checking for an
> AMD CPU. We can probably break it down into a stable-friendly fix that
> just jumps to out_flush_all on AMD CPUs, then the hardening on top.

Although it could be less crud if we just made svm_flush_tlb_gva() do
the fallback. kvm_hv_vcpu_flush_tlb() would need to key-off AMD CPU
though instead of the presence of flush_tlb_gva. Pick your poison, I
guess.