Re: [PATCH] KVM: do not prepare new memslot for KVM_MR_DELETE

[Yan Zhao <yan.y.zhao@intel.com>]

On Thu, Nov 10, 2022 at 01:53:47AM +0000, Sean Christopherson wrote:
> On Thu, Nov 10, 2022, Yan Zhao wrote:
> > kvm_prepare_memory_region() is not useful for KVM_MR_DELETE,
> > and each kvm_arch_prepare_memory_region() does nothing more than returning
> > 0 for KVM_MR_DELETE.
> 
> This is not true, s390 has an error path that fires in the DELETE case.
> 
> 	/* When we are protected, we should not change the memory slots */
> 	if (kvm_s390_pv_get_handle(kvm))
> 		return -EINVAL;
> 
> 	if (change == KVM_MR_DELETE || change == KVM_MR_FLAGS_ONLY)
> 		return 0;
> 
> 
> > So, just don't call into kvm_prepare_memory_region() to avoid unnecessary
> > error handling for KVM_MR_DELETE.
> 
> Even if the s390 case didn't exit, I would still prefer keeping the code as is.
> I agree that a "dummy" call is somewhat confusing, but skipping the arch call
> for one operation is equally confusing and could easily lead to future bugs,
> e.g. if the order of changes were reversed and an s390 developer wasn't aware
> that s390's arch hook would be bypassed on memslot deletion.
yes, you are right!