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: Wed, 22 May 2024 09:29:08 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>
CC: Sean Christopherson <seanjc@...gle.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 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.

I have the same feeling that the bug is probably not reproducible with latest
KVM code. And even when both ways are bug free, some VMs may still prefer to
fast zapping given it's fast.
So, I'm wondering if a cap in [1] is better.

[1] https://lore.kernel.org/kvm/20200713190649.GE29725@linux.intel.com/T/#mabc0119583dacf621025e9d873c85f4fbaa66d5c