linux-kernel - Re: [PATCH v19 038/130] KVM: TDX: create/destroy VM structure

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Message-ID: <58e0cf59-1397-44a3-a6a0-e26b2e51ba7b@intel.com>
Date: Thu, 4 Apr 2024 11:13:49 +1300
From: "Huang, Kai" <kai.huang@...el.com>
To: "Yamahata, Isaku" <isaku.yamahata@...el.com>, "Gao, Chao"
	<chao.gao@...el.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, Paolo Bonzini
	<pbonzini@...hat.com>, "Aktas, Erdem" <erdemaktas@...gle.com>, "Sean
 Christopherson" <seanjc@...gle.com>, Sagi Shahar <sagis@...gle.com>, "Chen,
 Bo2" <chen.bo@...el.com>, "Yuan, Hang" <hang.yuan@...el.com>, "Zhang, Tina"
	<tina.zhang@...el.com>, Sean Christopherson
	<sean.j.christopherson@...el.com>, "isaku.yamahata@...ux.intel.com"
	<isaku.yamahata@...ux.intel.com>
Subject: Re: [PATCH v19 038/130] KVM: TDX: create/destroy VM structure



On 22/03/2024 3:17 am, Yamahata, Isaku wrote:
>>> +
>>> +	for_each_online_cpu(i) {
>>> +		int pkg = topology_physical_package_id(i);
>>> +
>>> +		if (cpumask_test_and_set_cpu(pkg, packages))
>>> +			continue;
>>> +
>>> +		/*
>>> +		 * Program the memory controller in the package with an
>>> +		 * encryption key associated to a TDX private host key id
>>> +		 * assigned to this TDR.  Concurrent operations on same memory
>>> +		 * controller results in TDX_OPERAND_BUSY.  Avoid this race by
>>> +		 * mutex.
>>> +		 */
>>> +		mutex_lock(&tdx_mng_key_config_lock[pkg]);
>> the lock is superfluous to me. with cpu lock held, even if multiple CPUs try to
>> create TDs, the same set of CPUs (the first online CPU of each package) will be
>> selected to configure the key because of the cpumask_test_and_set_cpu() above.
>> it means, we never have two CPUs in the same socket trying to program the key,
>> i.e., no concurrent calls.
> Makes sense. Will drop the lock.

Hmm.. Skipping in cpumask_test_and_set_cpu() would result in the second 
TDH.MNG.KEY.CONFIG not being done for the second VM.  No?