linux-kernel - Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when loading the KVM intel kernel module

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Date: Fri, 19 Apr 2024 10:23:52 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: Tina Zhang <tina.zhang@...el.com>, Hang Yuan <hang.yuan@...el.com>, 
	Bo2 Chen <chen.bo@...el.com>, "sagis@...gle.com" <sagis@...gle.com>, 
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.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>, 
	"isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>
Subject: Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when
 loading the KVM intel kernel module

On Fri, Apr 19, 2024, Kai Huang wrote:
> On 19/04/2024 2:30 am, Sean Christopherson wrote:
> > No, that will deadlock as cpuhp_setup_state() does cpus_read_lock().
> 
> Right, but it takes cpus_read_lock()/unlock() internally.  I was talking
> about:
> 
> 	if (enable_tdx) {
> 		kvm_x86_virtualization_enable();
> 
> 		/*
> 		 * Unfortunately currently tdx_enable() internally has
> 		 * lockdep_assert_cpus_held().
> 		 */
> 		cpus_read_lock();
> 		tdx_enable();
> 		cpus_read_unlock();
> 	}

Ah.  Just have tdx_enable() do cpus_read_lock(), I suspect/assume the current
implemention was purely done in anticipation of KVM "needing" to do tdx_enable()
while holding cpu_hotplug_lock.

And tdx_enable() should also do its best to verify that the caller is post-VMXON:

	if (WARN_ON_ONCE(!(__read_cr4() & X86_CR4_VMXE)))
		return -EINVAL;

> > > Btw, why couldn't we do the 'system_state' check at the very beginning of
> > > this function?
> > 
> > We could, but we'd still need to check after, and adding a small bit of extra
> > complexity just to try to catch a very rare situation isn't worth it.
> > 
> > To prevent races, system_state needs to be check after register_syscore_ops(),
> > because only once kvm_syscore_ops is registered is KVM guaranteed to get notified
> > of a shutdown. >
> > And because the kvm_syscore_ops hooks disable virtualization, they should be called
> > after cpuhp_setup_state().  That's not strictly required, as the per-CPU
> > hardware_enabled flag will prevent true problems if the system enter shutdown
> > state before KVM reaches cpuhp_setup_state().
> > 
> > Hmm, but the same edge cases exists in the above flow.  If the system enters
> > shutdown _just_ after register_syscore_ops(), KVM would see that in system_state
> > and do cpuhp_remove_state(), i.e. invoke kvm_offline_cpu() and thus do a double
> > disable (which again is benign because of hardware_enabled).
> > 
> > Ah, but registering syscore ops before doing cpuhp_setup_state() has another race,
> > and one that could be fatal.  If the system does suspend+resume before the cpuhup
> > hooks are registered, kvm_resume() would enable virtualization.  And then if
> > cpuhp_setup_state() failed, virtualization would be left enabled.
> > 
> > So cpuhp_setup_state() *must* come before register_syscore_ops(), and
> > register_syscore_ops() *must* come before the system_state check.
> 
> OK.  I guess I have to double check here to completely understand the races.
> :-)
> 
> So I think we have consensus to go with the approach that shows in your
> second diff -- that is to always enable virtualization during module loading
> for all other ARCHs other than x86, for which we only always enables
> virtualization during module loading for TDX.

Assuming the other arch maintainers are ok with that approach.  If waiting until
a VM is created is desirable for other architectures, then we'll need to figure
out a plan b.  E.g. KVM arm64 doesn't support being built as a module, so enabling
hardware during initialization would mean virtualization is enabled for any kernel
that is built with CONFIG_KVM=y.

Actually, duh.  There's absolutely no reason to force other architectures to
choose when to enable virtualization.  As evidenced by the massaging to have x86
keep enabling virtualization on-demand for !TDX, the cleanups don't come from
enabling virtualization during module load, they come from registering cpuup and
syscore ops when virtualization is enabled.

I.e. we can keep kvm_usage_count in common code, and just do exactly what I
proposed for kvm_x86_enable_virtualization().

I have patches to do this, and initial testing suggests they aren't wildly
broken.  I'll post them soon-ish, assuming nothing pops up in testing.  They are
clean enough that they can land in advance of TDX, e.g. in kvm-coco-queue even
before other architectures verify I didn't break them.

> Then how about "do kvm_x86_virtualization_enable()  within
> late_hardware_setup() in kvm_x86_vendor_init()"  vs "do
> kvm_x86_virtualization_enable() in TDX-specific code after
> kvm_x86_vendor_init()"?
> 
> Which do you prefer?

The latter, assuming it doesn't make the TDX code more complex than it needs to
be.  The fewer kvm_x86_ops hooks, the better.