Re: [PATCH v3 04/37] KVM: nVMX: Invalidate all roots when emulating INVVPID without EPT

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

> On Mon, Mar 23, 2020 at 04:34:17PM +0100, Vitaly Kuznetsov wrote:
>> Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:
>> 
>> > From: Junaid Shahid <junaids@xxxxxxxxxx>
>> >
>> > Free all roots when emulating INVVPID for L1 and EPT is disabled, as
>> > outstanding changes to the page tables managed by L1 need to be
>> > recognized.  Because L1 and L2 share an MMU when EPT is disabled, and
>> > because VPID is not tracked by the MMU role, all roots in the current
>> > MMU (root_mmu) need to be freed, otherwise a future nested VM-Enter or
>> > VM-Exit could do a fast CR3 switch (without a flush/sync) and consume
>> > stale SPTEs.
>> >
>> > Fixes: 5c614b3583e7b ("KVM: nVMX: nested VPID emulation")
>> > Signed-off-by: Junaid Shahid <junaids@xxxxxxxxxx>
>> > [sean: ported to upstream KVM, reworded the comment and changelog]
>> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>> > ---
>> >  arch/x86/kvm/vmx/nested.c | 14 ++++++++++++++
>> >  1 file changed, 14 insertions(+)
>> >
>> > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>> > index 9624cea4ed9f..bc74fbbf33c6 100644
>> > --- a/arch/x86/kvm/vmx/nested.c
>> > +++ b/arch/x86/kvm/vmx/nested.c
>> > @@ -5250,6 +5250,20 @@ static int handle_invvpid(struct kvm_vcpu *vcpu)
>> >  		return kvm_skip_emulated_instruction(vcpu);
>> >  	}
>> >  
>> > +	/*
>> > +	 * Sync the shadow page tables if EPT is disabled, L1 is invalidating
>> > +	 * linear mappings for L2 (tagged with L2's VPID).  Free all roots as
>> > +	 * VPIDs are not tracked in the MMU role.
>> > +	 *
>> > +	 * Note, this operates on root_mmu, not guest_mmu, as L1 and L2 share
>> > +	 * an MMU when EPT is disabled.
>> > +	 *
>> > +	 * TODO: sync only the affected SPTEs for INVDIVIDUAL_ADDR.
>> > +	 */
>> > +	if (!enable_ept)
>> > +		kvm_mmu_free_roots(vcpu, &vcpu->arch.root_mmu,
>> > +				   KVM_MMU_ROOTS_ALL);
>> > +
>> 
>> This is related to my remark on the previous patch; the comment above
>> makes me think I'm missing something obvious, enlighten me please)
>> 
>> My understanding is that L1 and L2 will share arch.root_mmu not only
>> when EPT is globally disabled, we seem to switch between
>> root_mmu/guest_mmu only when nested_cpu_has_ept(vmcs12) but different L2
>> guests may be different on this. Do we need to handle this somehow?
>
> guest_mmu is used iff nested EPT is enabled, which requires enable_ept=1.
> enable_ept is global and cannot be changed without reloading kvm_intel.
>
> This most definitely over-invalidates, e.g. it blasts away L1's page
> tables.  But, fixing that requires tracking VPID in mmu_role and/or adding
> support for using guest_mmu when L1 isn't using TDP, i.e. nested EPT is
> disabled.  Assuming the vast majority of nested deployments enable EPT in
> L0, the cost of both options likely outweighs the benefits.
>

Yes but my question rather was: what if global 'enable_ept' is true but
nested EPT is not being used by L1, don't we still need to do
kvm_mmu_free_roots(&vcpu->arch.root_mmu) here?

-- 
Vitaly