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

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Message-ID: <Z-WqIxO1XfFAmchX@google.com>
Date: Thu, 27 Mar 2025 12:42:27 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Nikunj A Dadhania <nikunj@....com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Jim Mattson <jmattson@...gle.com>, Maxim Levitsky <mlevitsk@...hat.com>, 
	Vitaly Kuznetsov <vkuznets@...hat.com>, Rik van Riel <riel@...riel.com>, 
	Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Manali Shukla <manali.shukla@....com>, 
	santosh.shukla@....com
Subject: Re: [RFC PATCH 01/24] KVM: VMX: Generalize VPID allocation to be vendor-neutral

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).