linux-kernel - Re: [PATCH 02/16] KVM: x86/mmu: Introduce a slot flag to zap only slot leafs on slot deletion

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Date: Tue, 21 May 2024 19:31:31 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Kai Huang <kai.huang@...el.com>, 
	Rick P Edgecombe <rick.p.edgecombe@...el.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>, 
	"sagis@...gle.com" <sagis@...gle.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Erdem Aktas <erdemaktas@...gle.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>
Subject: Re: [PATCH 02/16] KVM: x86/mmu: Introduce a slot flag to zap only
 slot leafs on slot deletion

On Wed, May 22, 2024, Yan Zhao wrote:
> On Fri, May 17, 2024 at 05:30:50PM +0200, Paolo Bonzini wrote:
> > On 5/16/24 01:20, Sean Christopherson wrote:
> > > Hmm, a quirk isn't a bad idea.  It suffers the same problems as a memslot flag,
> > > i.e. who knows when it's safe to disable the quirk, but I would hope userspace
> > > would be much, much cautious about disabling a quirk that comes with a massive
> > > disclaimer.
> > > 
> > > Though I suspect Paolo will shoot this down too 😉
> > 
> > Not really, it's probably the least bad option.  Not as safe as keying it
> > off the new machine types, but less ugly.
> A concern about the quirk is that before identifying the root cause of the
> issue, we don't know which one is a quirk, fast zapping all TDPs or slow zapping
> within memslot range.

The quirk is specifically that KVM zaps SPTEs that aren't related to the memslot
being deleted/moved.  E.g. the issue went away if KVM zapped a rather arbitrary
set of SPTEs.  IIRC, there was a specific gfn range that was "problematic", but
we never figured out the correlation between the problematic range and the memslot
being deleted.

Disabling the quirk would allow KVM to choose between a slow/precise/partial zap,
and full/fast zap.