Subject: Re: [RFC PATCH v11 01/29] KVM: Wrap kvm_gfn_range.pte in a per-action union

From: Sean Christopherson <[email-masked]>
Date: Wed, 19 Jul 2023 08:39:09 -0700

On Wed, Jul 19, 2023, Jarkko Sakkinen wrote:
> On Wed Jul 19, 2023 at 2:44 AM EEST, Sean Christopherson wrote:
> >  	/* Huge pages aren't expected to be modified without first being zapped. */
> > -	WARN_ON(pte_huge(range->pte) || range->start + 1 != range->end);
> > +	WARN_ON(pte_huge(range->arg.pte) || range->start + 1 != range->end);
> 
> Not familiar with this code. Just checking whether whether instead
> pr_{warn,err}()

The "full" WARN is desirable, this is effecitvely an assert on the contract between
the primary MMU, generic KVM code, and x86's TDP MMU.  The .change_pte() mmu_notifier
callback doesn't allow for hugepages, i.e. it's a (likely fatal) kernel bug if a
hugepage is encountered at this point.  Ditto for the "start + 1 == end" check,
if that fails then generic KVM likely has a fatal bug.

> combined with return false would be a more graceful option?

The return value communicates whether or not a TLB flush is needed, not whether
or not the operation was successful, i.e. there is no way to cancel the unexpected
PTE change.