Re: [PATCH 2/7] KVM: X86: Synchronize the shadow pagetable before link it

[Maxim Levitsky <mlevitsk@redhat.com>]

On Mon, 2021-09-13 at 20:49 +0000, Sean Christopherson wrote:
> On Mon, Sep 13, 2021, Maxim Levitsky wrote:
> > On Thu, 2021-09-02 at 23:40 +0000, Sean Christopherson wrote:
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 4853c033e6ce..03293cd3c7ae 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -2143,8 +2143,10 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
> > >  			kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
> > >  		}
> > > 
> > > -		if (sp->unsync_children)
> > > -			kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
> > > +		if (sp->unsync_children) {
> > > +			kvm_make_all_cpus_request(KVM_REQ_MMU_SYNC, vcpu);
> > 
> > I don't know the KVM mmu well so I miss something here most likely,
> > but why to switch to kvm_make_all_cpus_request?
> > 
> > MMU is shared by all VCPUs, and the process of its syncing should also do
> > remote TLB flushes when needed?
> > 
> > Another thing I don't fully understand is why this patch is needed. If we
> > link an SP which has unsync children, we raise KVM_REQ_MMU_SYNC, which I
> > think means that *this* vCPU will sync the whole MMU on next guest entry,
> > including these unsync child SPs. Could you explain this?
> 
> Answering all three questions at once, the problem is that KVM links in a new SP
> that points at unsync'd SPs _before_ servicing KVM_REQ_MMU_SYNC.  While the vCPU
> is guaranteed to service KVM_REQ_MMU_SYNC before entering the guest, that doesn't
> hold true for other vCPUs.  As a result, there's a window where a different vCPU
> can consume the stale, unsync SP via the new SP.
> 
Thank you, now I understand!

Best regards,
	Maxim Levitsky