linux-kernel - Re: [PATCH v3 04/21] x86/virt/tdx: Add skeleton for detecting and initializing TDX on demand

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Date:   Thu, 28 Apr 2022 16:53:59 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v3 04/21] x86/virt/tdx: Add skeleton for detecting and
 initializing TDX on demand

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.