Re: [PATCH 2/2] KVM: nVMX: Emulate guest TLB flush on nested VM-Enter with new vpid12

[Lai Jiangshan]

On Thu, Nov 25, 2021 at 9:49 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> Fully emulate a guest TLB flush on nested VM-Enter which changes vpid12,
> i.e. L2's VPID, instead of simply doing INVVPID to flush real hardware's
> TLB entries for vpid02.  From L1's perspective, changing L2's VPID is
> effectively a TLB flush unless "hardware" has previously cached entries
> for the new vpid12.  Because KVM tracks only a single vpid12, KVM doesn't
> know if the new vpid12 has been used in the past and so must treat it as
> a brand new, never been used VPID, i.e. must assume that the new vpid12
> represents a TLB flush from L1's perspective.
>
> For example, if L1 and L2 share a CR3, the first VM-Enter to L2 (with a
> VPID) is effectively a TLB flush as hardware/KVM has never seen vpid12
> and thus can't have cached entries in the TLB for vpid12.
>
> Reported-by: Lai Jiangshan <jiangshanlai+lkml@xxxxxxxxx>
> Fixes: 5c614b3583e7 ("KVM: nVMX: nested VPID emulation")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/nested.c | 37 +++++++++++++++++--------------------
>  1 file changed, 17 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 2ef1d5562a54..dafe5881ae51 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -1162,29 +1162,26 @@ static void nested_vmx_transition_tlb_flush(struct kvm_vcpu *vcpu,
>         WARN_ON(!enable_vpid);
>
>         /*
> -        * If VPID is enabled and used by vmc12, but L2 does not have a unique
> -        * TLB tag (ASID), i.e. EPT is disabled and KVM was unable to allocate
> -        * a VPID for L2, flush the current context as the effective ASID is
> -        * common to both L1 and L2.
> -        *
> -        * Defer the flush so that it runs after vmcs02.EPTP has been set by
> -        * KVM_REQ_LOAD_MMU_PGD (if nested EPT is enabled) and to avoid
> -        * redundant flushes further down the nested pipeline.
> -        *
> -        * If a TLB flush isn't required due to any of the above, and vpid12 is
> -        * changing then the new "virtual" VPID (vpid12) will reuse the same
> -        * "real" VPID (vpid02), and so needs to be flushed.  There's no direct
> -        * mapping between vpid02 and vpid12, vpid02 is per-vCPU and reused for
> -        * all nested vCPUs.  Remember, a flush on VM-Enter does not invalidate
> -        * guest-physical mappings, so there is no need to sync the nEPT MMU.
> +        * VPID is enabled and in use by vmcs12.  If vpid12 is changing, then
> +        * emulate a guest TLB flush as KVM does not track vpid12 history nor
> +        * is the VPID incorporated into the MMU context.  I.e. KVM must assume
> +        * that the new vpid12 has never been used and thus represents a new
> +        * guest ASID that cannot have entries in the TLB.
>          */
> -       if (!nested_has_guest_tlb_tag(vcpu)) {
> -               kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
> -       } else if (is_vmenter &&
> -                  vmcs12->virtual_processor_id != vmx->nested.last_vpid) {
> +       if (is_vmenter && vmcs12->virtual_processor_id != vmx->nested.last_vpid) {
>                 vmx->nested.last_vpid = vmcs12->virtual_processor_id;

How about when vmx->nested.last_vpid == vmcs12->virtual_processor_id == 0?

I think KVM_REQ_TLB_FLUSH_GUEST is needed in this case too.

> -               vpid_sync_context(nested_get_vpid02(vcpu));
> +               kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu);
> +               return;
>         }
> +
> +       /*
> +        * If VPID is enabled, used by vmc12, and vpid12 is not changing but
> +        * does not have a unique TLB tag (ASID), i.e. EPT is disabled and
> +        * KVM was unable to allocate a VPID for L2, flush the current context
> +        * as the effective ASID is common to both L1 and L2.
> +        */
> +       if (!nested_has_guest_tlb_tag(vcpu))
> +               kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
>  }
>
>  static bool is_bitwise_subset(u64 superset, u64 subset, u64 mask)
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>