Re: [PATCH v5 04/13] KVM: x86: Move n_memslots_pages recalc to kvm_arch_prepare_memory_region()

[Maciej S. Szmigiero]

On 20.10.2021 00:38, Sean Christopherson wrote:
> On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote:
> > From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>
> >
> > This allows us to return a proper error code in case we spot an underflow.
> >
> > Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/x86.c | 49 ++++++++++++++++++++++++++--------------------
> >   1 file changed, 28 insertions(+), 21 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 97d86223427d..0fffb8414009 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -11511,9 +11511,23 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
> >   				const struct kvm_userspace_memory_region *mem,
> >   				enum kvm_mr_change change)
> >   {
> > -	if (change == KVM_MR_CREATE || change == KVM_MR_MOVE)
> > -		return kvm_alloc_memslot_metadata(kvm, new,
> > -						  mem->memory_size >> PAGE_SHIFT);
> > +	if (change == KVM_MR_CREATE || change == KVM_MR_MOVE) {
> > +		int ret;
> > +
> > +		ret = kvm_alloc_memslot_metadata(kvm, new,
> > +						 mem->memory_size >> PAGE_SHIFT);
> > +		if (ret)
> > +			return ret;
> > +
> > +		if (change == KVM_MR_CREATE)
> > +			kvm->arch.n_memslots_pages += new->npages;
> > +	} else if (change == KVM_MR_DELETE) {
> > +		if (WARN_ON(kvm->arch.n_memslots_pages < old->npages))
> > +			return -EIO;
>
> This is not worth the churn.  In a way, it's worse because userspace can spam
> the living snot out of the kernel log by retrying the ioctl().
>
> Since underflow can happen if and only if there's a KVM bug, and a pretty bad one
> at that, just make the original WARN_ON a KVM_BUG_ON.  That will kill the VM and
> also provide the WARN_ON_ONCE behavior that we probably want.

Will do.

Thanks,
Maciej