linux-kernel - Re: [PATCH v8 12/16] x86/virt/tdx: Designate the global KeyID and configure the TDX module

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Message-ID: <755f94aa-a0cc-b7d6-ce8a-a81ff4f598da@intel.com>
Date:   Tue, 10 Jan 2023 08:25:42 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "Luck, Tony" <tony.luck@...el.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v8 12/16] x86/virt/tdx: Designate the global KeyID and
 configure the TDX module

On 1/10/23 02:48, Huang, Kai wrote:
>>>
>>> +   /*
>>> +    * Use the first private KeyID as the global KeyID, and pass
>>> +    * it along with the TDMRs to the TDX module.
>>> +    */
>>> +   ret = config_tdx_module(&tdmr_list, tdx_keyid_start);
>>> +   if (ret)
>>> +           goto out_free_pamts;
>> This is "consuming" tdx_keyid_start.  Does it need to get incremented
>> since the first guest can't use this KeyID now?
> 
> It depends on how we treat 'tdx_keyid_start'.  If it means the first _usable_
> KeyID for KVM, then we should increase it; but if it only used for the hardware-
> enabled TDX KeyID range, then we don't need to increase it.
> 
> Currently it is marked as __ro_after_init so my intention is the latter (also in
> the spirit of keeping this series minimal).
> 
> Eventually we will need to have functions to allocate/free TDX KeyIDs anyway for
> KVM, but in that we can just treat 'tdx_keyid_start + 1' as the first usable
> KeyID.

So, basically, you're going to depend on the KVM code (which isn't in
this series) to magically know exactly what this series did?  Then,
you're expecting that this code will never change in a way that breaks
this random KVM code?

That's frankly awful.

Make the variable read/write.  Call it tdx_guest_keyid_start, and
increment it when you make a keyid unavailable for guest use.