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Message-ID: <572c0f5ce627384b6441b64b9fa036b202d430b7.camel@intel.com>
Date: Fri, 23 May 2025 11:31:10 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
<pbonzini@...hat.com>, "vipinsh@...gle.com" <vipinsh@...gle.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/3] KVM: x86: Use kvzalloc() to allocate VM struct
On Thu, 2025-05-22 at 06:40 -0700, Sean Christopherson wrote:
> On Wed, May 21, 2025, Kai Huang wrote:
> > On Wed, 2025-05-21 at 10:12 -0700, Sean Christopherson wrote:
> > > > e.g., if we export kvm_x86_ops, we could unwind them.
> > >
> > > Maaaybe. I mean, yes, we could fully unwind kvm_x86_ops, but doing so would make
> > > the overall code far more brittle. E.g. simply updating kvm_x86_ops won't suffice,
> > > as the static_calls also need to be patched, and we would have to be very careful
> > > not to touch anything in kvm_x86_ops that might have been consumed between here
> > > and the call to tdx_bringup().
> >
> > Right. Maybe exporting kvm_ops_update() is better.
>
> A bit, but KVM would still need to be careful not to modify the parts of
> vt_x86_ops that have already been consumed.
>
> While I agree that leaving TDX breadcrumbs in kvm_x86_ops when TDX is disabled is
> undesirable, the behavior is known, i.e. we know exactly what TDX state is being
> left behind. And failure to load the TDX Module should be very, very rare for
> any setup that is actually trying to enable TDX.
This is true. Agreed.
>
> Whereas providing a way to modify kvm_x86_ops creates the possibility for "bad"
> updates. KVM's initialization code is a lot like the kernel's boot code (and
> probably most bootstrapping code): it's inherently fragile because avoiding
> dependencies is practically impossible.
>
> E.g. I ran into a relevant ordering problem[*] just a few days ago, where checking
> for VMX capabilities during PMU initialization always failed because the VMCS
> config hadn't yet been parsed. Those types of bugs are especially dangerous
> because they're very easy to overlook when modifying existing code, e.g. the
> only sign that anything is broken is an optional feature being missing.
>
> [*] https://lore.kernel.org/all/aCU2YEpU0dOk7RTk@google.com
Right. No argument around this. I agree if there are multiple features wanting
to update then it could be dangerous. Thanks for the insight :-)
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