Re: [PATCH v3 04/28] KVM: x86/mmu: Formalize TDP MMU's (unintended?) deferred TLB flush logic

[Mingwei Zhang <mizhang@google.com>]

On Thu, Mar 03, 2022, Sean Christopherson wrote:
> On Wed, Mar 02, 2022, Mingwei Zhang wrote:
> > On Sat, Feb 26, 2022, Sean Christopherson wrote:
> > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > > index 12866113fb4f..e35bd88d92fd 100644
> > > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > > @@ -93,7 +93,15 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
> > >  	list_del_rcu(&root->link);
> > >  	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> > >  
> > > -	zap_gfn_range(kvm, root, 0, -1ull, false, false, shared);
> > > +	/*
> > > +	 * A TLB flush is not necessary as KVM performs a local TLB flush when
> > > +	 * allocating a new root (see kvm_mmu_load()), and when migrating vCPU
> > > +	 * to a different pCPU.  Note, the local TLB flush on reuse also
> > > +	 * invalidates any paging-structure-cache entries, i.e. TLB entries for
> > > +	 * intermediate paging structures, that may be zapped, as such entries
> > > +	 * are associated with the ASID on both VMX and SVM.
> > > +	 */
> > > +	(void)zap_gfn_range(kvm, root, 0, -1ull, false, false, shared);
> > 
> > Understood that we could avoid the TLB flush here. Just curious why the
> > "(void)" is needed here? Is it for compile time reason?
> 
> Nope, no functional purpose, though there might be some "advanced" warning or
> static checkers that care.
> 
> The "(void)" is to communicate to human readers that the result is intentionally
> ignored, e.g. to reduce the probability of someone "fixing" the code by acting on
> the result of zap_gfn_range().  The comment should suffice, but it's nice to have
> the code be self-documenting as much as possible.

Right, I got the point. Thanks.

Coming back. It seems that I pretended to understand that we should
avoid the TLB flush without really knowing why.

I mean, leaving (part of the) stale TLB entries unflushed will still be
dangerous right? Or am I missing something that guarantees to flush the
local TLB before returning to the guest? For instance,
kvm_mmu_{re,}load()?