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Date:   Tue, 5 Dec 2023 08:57:33 -0800
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Subject: Re: [PATCH v15 22/23] x86/mce: Improve error log of kernel space TDX
 #MC due to erratum
From:   Sean Christopherson <seanjc@google.com>
To:     Tony Luck <tony.luck@intel.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Kai Huang <kai.huang@intel.com>,
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On Tue, Dec 05, 2023, Tony Luck wrote:
> >> Fine.  This doesn't need to change ... until you load TDX.  Once you
> >> initialize the TDX module, no more out-of-tree VMMs for you.
> >
> > It's not just out-of-tree hypervisors, which IMO should be little more than an
> > afterthought.  The other more important issue is that being post-VMXON blocks INIT,
> 
> Does that make CPU offline a one-way process? Linux uses INIT to bring a CPU back
> online again.

No, KVM does VMXOFF on the CPU being offlined, and then VMXON if/when the CPU is
onlined again.  This also handles secondary CPUs for suspend/resume (the primary
CPU hooks .suspend() and .resume()).

static int kvm_offline_cpu(unsigned int cpu)
{
	mutex_lock(&kvm_lock);
	if (kvm_usage_count)
		hardware_disable_nolock(NULL);
	mutex_unlock(&kvm_lock);
	return 0;
}


static int kvm_online_cpu(unsigned int cpu)
{
	int ret = 0;

	/*
	 * Abort the CPU online process if hardware virtualization cannot
	 * be enabled. Otherwise running VMs would encounter unrecoverable
	 * errors when scheduled to this CPU.
	 */
	mutex_lock(&kvm_lock);
	if (kvm_usage_count)
		ret = __hardware_enable_nolock();
	mutex_unlock(&kvm_lock);
	return ret;
}