Re: [RFC PATCH 01/24] KVM: VMX: Generalize VPID allocation to be vendor-neutral

[Sean Christopherson]

On Thu, Mar 27, 2025, Yosry Ahmed wrote:
> On Thu, Mar 27, 2025 at 10:58:31AM +0000, Nikunj A Dadhania wrote:
> > > +unsigned int kvm_tlb_tags_alloc(struct kvm_tlb_tags *tlb_tags)
> > > +{
> > > +	unsigned int tag;
> > > +
> > > +	spin_lock(&tlb_tags->lock);
> > > +	tag = find_next_zero_bit(tlb_tags->bitmap, tlb_tags->max + 1,
> > > +				 tlb_tags->min);
> > > +	if (tag <= tlb_tags->max)
> > > +		__set_bit(tag, tlb_tags->bitmap);
> > > +	else
> > > +		tag = 0;
> > 
> > In the event that the KVM runs out of tags, adding WARN_ON_ONCE() here will
> > help debugging.
> 
> Yeah I wanted to do that, but we do not currently WARN in VMX if we run
> out of VPIDs. I am fine with doing adding it if others are. My main
> concern was if there's some existing use case that routinely runs out of
> VPIDs (although I cannot imagine one).

No WARNs, it would be userspace triggerable (hello, syzkaller).  If we really
want to harden things against performance issues due to unexpected VPID/ASID
allocation, I would rather do something like add a knob to fail VM or vCPU
creation if allocation fails (nested would just have to suffer).