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Date: Wed, 8 May 2024 07:38:11 -0700
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Subject: Re: [PATCH v5 07/17] KVM: x86: add fields to struct kvm_arch for CoCo features
From: Sean Christopherson <seanjc@google.com>
To: Rick P Edgecombe <rick.p.edgecombe@intel.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>, "pbonzini@redhat.com" <pbonzini@redhat.com>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Yan Y Zhao <yan.y.zhao@intel.com>, 
	"michael.roth@amd.com" <michael.roth@amd.com>, Isaku Yamahata <isaku.yamahata@intel.com>
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On Wed, May 08, 2024, Rick P Edgecombe wrote:
> On Tue, 2024-05-07 at 17:21 -0700, Sean Christopherson wrote:
> > > Can you elaborate on the reason for a per-memslot flag? We are discus=
sing
> > > this
> > > design point internally, and also the intersection with the previous
> > > attempts to
> > > do something similar with a per-vm flag[0].
> > >=20
> > > I'm wondering if the intention is to try to make a memslot flag, so i=
t can
> > > be
> > > expanded for the normal VM usage.
> >=20
> > Sure, I'll go with that answer.=C2=A0 Like I said, off-the-cuff.
> >=20
> > There's no concrete motiviation, it's more that _if_ we're going to exp=
ose
> > a knob to userspace, then I'd prefer to make it as precise as possible =
to
> > minimize the changes of KVM ending up back in ABI hell again.
> >=20
> > > Because the discussion on the original attempts, it seems safer to ke=
ep this
> > > behavior more limited (TDX only) for now.=C2=A0 And for TDX's usage a=
 struct kvm
> > > bool fits best because all memslots need to be set to zap_leafs_only =
=3D true,
> > > anyway.
> >=20
> > No they don't.=C2=A0 They might be set that way in practice for QEMU, b=
ut it's
> > not strictly required.=C2=A0 E.g. nothing would prevent a VMM from expo=
sing a
> > shared- only memslot to a guest.=C2=A0 The memslots that burned KVM the=
 first
> > time around were related to VFIO devices, and I wouldn't put it past
> > someone to be crazy enough
> > to expose an passhtrough an untrusted device to a TDX guest.
>=20
> Ok, thanks for clarification. So it's more of a strategic thing to move m=
ore
> zapping logic into userspace so the logic can change without introducing =
kernel
> regressions.

You're _really_ reading too much into my suggestion.  As above, my suggesti=
on
was very spur of the momemnt.  I haven't put much thought into the tradeoff=
s and
side effects.