Re: [PATCH v3 33/37] KVM: nVMX: Skip MMU sync on nested VMX transition when possible

[Paolo Bonzini <pbonzini@redhat.com>]

On 20/03/20 22:28, Sean Christopherson wrote:
> Skip the MMU sync when reusing a cached root if EPT is enabled or L1
> enabled VPID for L2.
> 
> If EPT is enabled, guest-physical mappings aren't flushed even if VPID
> is disabled, i.e. L1 can't expect stale TLB entries to be flushed if it
> has enabled EPT and L0 isn't shadowing PTEs (for L1 or L2) if L1 has
> EPT disabled.
> 
> If VPID is enabled (and EPT is disabled), then L1 can't expect stale TLB
> entries to be flushed (for itself or L2).
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>

Great, just a small rephrasing here and there:

/*
 * Returns true if the MMU needs to be sync'd on nested VM-Enter/VM-Exit.
 * tl;dr: the MMU needs a sync if L0 is using shadow paging and L1 didn't
 * enable VPID for L2 (implying it expects a TLB flush on VMX transitions).
 * Here's why.
 *
 * If EPT is enabled by L0 a sync is never needed:
 * - if it is disabled by L1, then L0 is not shadowing L1 or L2 PTEs, there
 *   cannot be unsync'd SPTEs for either L1 or L2.
 *
 * - if it is also enabled by L1, then L0 doesn't need to sync on VM-Enter
 *   VM-Enter as VM-Enter isn't required to invalidate guest-physical mappings
 *   (irrespective of VPID), i.e. L1 can't rely on the (virtual) CPU to flush
 *   stale guest-physical mappings for L2 from the TLB.  And as above, L0 isn't
 *   shadowing L1 PTEs so there are no unsync'd SPTEs to sync on VM-Exit.
 *
 * If EPT is disabled by L0:
 * - if VPID is enabled by L1 (for L2), the situation is similar to when L1
 *   enables EPT: L0 doesn't need to sync as VM-Enter and VM-Exit aren't
 *   required to invalidate linear mappings (EPT is disabled so there are
 *   no combined or guest-physical mappings), i.e. L1 can't rely on the
 *   (virtual) CPU to flush stale linear mappings for either L2 or itself (L1).
 *
 * - however if VPID is disabled by L1, then a sync is needed as L1 expects all
 *   linear mappings (EPT is disabled so there are no combined or guest-physical
 *   mappings) to be invalidated on both VM-Enter and VM-Exit.
 *
 * Note, this logic is subtly different than nested_has_guest_tlb_tag(), which
 * additionally checks that L2 has been assigned a VPID (when EPT is disabled).
 * Whether or not L2 has been assigned a VPID by L0 is irrelevant with respect
 * to L1's expectations, e.g. L0 needs to invalidate hardware TLB entries if L2
 * doesn't have a unique VPID to prevent reusing L1's entries (assuming L1 has
 * been assigned a VPID), but L0 doesn't need to do a MMU sync because L1
 * doesn't expect stale (virtual) TLB entries to be flushed, i.e. L1 doesn't
 * know that L0 will flush the TLB and so L1 will do INVVPID as needed to flush
 * stale TLB entries, at which point L0 will sync L2's MMU.
 */

Paolo