Re: [PATCH v10 4/8] KVM: mmu: Improve handling of non-refcounted pfns

[Sean Christopherson]

On Fri, Feb 23, 2024, Paolo Bonzini wrote:
> On 2/21/24 08:25, David Stevens wrote:
> > +	} else if (!kfp->refcounted_page &&
> > +		   !kfp->guarded_by_mmu_notifier &&
> > +		   !allow_unsafe_mappings) {
> > +		r = -EFAULT;
> 
> Why is allow_unsafe_mappings desirable at all?

It's for use cases where memory is hidden from the kernel and managed by userspace,
e.g. where AWS uses /dev/mem (I think) to map guest memory.  From a kernel
perspective, that is unsafe because KVM won't do the right thing if userspace
unmaps memory while it is exposed to L2 via a pfn in vmcs02.

I suggested allow_unsafe_mappings as a way to make upstream KVM safe by default,
without completely breaking support for AWS and friends.