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Message-ID: <0aa81fd0-a491-847d-9fc6-4b853f2cf7b4@intel.com>
Date:   Thu, 28 Apr 2022 16:53:59 -0700
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Subject: Re: [PATCH v3 04/21] x86/virt/tdx: Add skeleton for detecting and
 initializing TDX on demand
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To:     Kai Huang <kai.huang@intel.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     seanjc@google.com, pbonzini@redhat.com, len.brown@intel.com,
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        reinette.chatre@intel.com, dan.j.williams@intel.com,
        peterz@infradead.org, ak@linux.intel.com,
        kirill.shutemov@linux.intel.com,
        sathyanarayanan.kuppuswamy@linux.intel.com,
        isaku.yamahata@intel.com
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From:   Dave Hansen <dave.hansen@intel.com>
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On 4/28/22 16:44, Kai Huang wrote:
>> Just like the SME test, it doesn't even need to be precise.  It just
>> needs to be 100% accurate in that it is *ALWAYS* set for any system that
>> might have dirtied cache aliases.
>>
>> I'm not sure why you are so fixated on SEAMRR specifically for this.
> I see.  I think I can simply use MTRR.SEAMRR bit check.  If CPU supports SEAMRR,
> then basically it supports MKTME.
> 
> Is this look good for you?

Sure, fine, as long as it comes with a coherent description that
explains why the check is good enough.

>>> "During initializing the TDX module, one step requires some SEAMCALL must be
>>> done on all logical cpus enabled by BIOS, otherwise a later step will fail. 
>>> Disable CPU hotplug during the initialization process to prevent any CPU going
>>> offline during initializing the TDX module.  Note it is caller's responsibility
>>> to guarantee all BIOS-enabled CPUs are in cpu_present_mask and all present CPUs
>>> are online."
>> But, what if a CPU went offline just before this lock was taken?  What
>> if the caller make sure all present CPUs are online, makes the call,
>> then a CPU is taken offline.  The lock wouldn't do any good.
>>
>> What purpose does the lock serve?
> I thought cpus_read_lock() can prevent any CPU from going offline, no?

It doesn't prevent squat before the lock is taken, though.