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 05:33:08PM +0100, Vitaly Kuznetsov wrote:
>> 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?
>
> No, because L0 isn't shadowing the L1->L2 page tables, i.e. there can't be
> unsync'd SPTEs for L2.  The vpid_sync_*() above flushes the TLB for L2's
> effective VPID, which is all that's required.

Ah, stupid me, it's actually EPT and not nested EPT which we care about
here. Thank you for the clarification!

Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

-- 
Vitaly