linux-kernel - Re: [PATCH 0/5] Introduce a quirk to control memslot zap behavior

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Date: Thu, 20 Jun 2024 17:00:50 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: Yan Y Zhao <yan.y.zhao@...el.com>, Kai Huang <kai.huang@...el.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>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>
Subject: Re: [PATCH 0/5] Introduce a quirk to control memslot zap behavior

On Thu, Jun 20, 2024, Rick P Edgecombe wrote:
> On Tue, 2024-06-18 at 07:34 -0700, Sean Christopherson wrote:
> > There's also option:
> > 
> >             c) Init disabled_quirks based on VM type.
> > 
> > I.e. let userspace enable the quirk.  If the VMM wants to shoot its TDX VM
> > guests, then so be it.  That said, I don't like this option because it
> > would create a very bizarre ABI.
> 
> I think we actually need to force it on for TDX because kvm_mmu_zap_all_fast()
> only zaps the direct (shared) root. If userspace decides to not enable the
> quirk, mirror/private memory will not be zapped on memslot deletion. Then later
> if there is a hole punch it will skip zapping that range because there is no
> memslot. Then won't it let the pages get freed while they are still mapped in
> the TD?
> 
> If I got that right (not 100% sure on the gmem hole punch page freeing), I think
> KVM needs to force the behavior for TDs.

What I was suggesting is that we condition the skipping of the mirror/private
EPT pages tables on the quirk, i.e. zap *everything* for TDX VMs if the quirk is
enabled.  Hence the very bizarre ABI.