On Thu, Nov 25, 2021, Lai Jiangshan wrote:
> On Thu, Nov 25, 2021 at 9:49 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > 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.
That's handled by code that's just out of sight in this diff. The first check in
nested_vmx_transition_tlb_flush() is to see if vmcs12 has VPID enabled. If the
code in this patch is reached, vmcs12->virtual_processor_id is guaranteed to be
non-zero as VM-Enter fails if VPID is enabled but VPID==0.
static void nested_vmx_transition_tlb_flush(struct kvm_vcpu *vcpu,
struct vmcs12 *vmcs12,
bool is_vmenter)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
/*
* If vmcs12 doesn't use VPID, L1 expects linear and combined mappings
* for *all* contexts to be flushed on VM-Enter/VM-Exit, i.e. it's a
* full TLB flush from the guest's perspective. This is required even
* if VPID is disabled in the host as KVM may need to synchronize the
* MMU in response to the guest TLB flush.
*
* Note, using TLB_FLUSH_GUEST is correct even if nested EPT is in use.
* EPT is a special snowflake, as guest-physical mappings aren't
* flushed on VPID invalidations, including VM-Enter or VM-Exit with
* VPID disabled. As a result, KVM _never_ needs to sync nEPT
* entries on VM-Enter because L1 can't rely on VM-Enter to flush
* those mappings.
*/
if (!nested_cpu_has_vpid(vmcs12)) {
kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu);
return;
}
...
}
