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

From: Dave Hansen <dave.hansen@intel.com>
To: "Huang, Kai" <kai.huang@intel.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc: "Luck, Tony" <tony.luck@intel.com>,
	"bagasdotme@gmail.com" <bagasdotme@gmail.com>,
	"ak@linux.intel.com" <ak@linux.intel.com>,
	"Wysocki, Rafael J" <rafael.j.wysocki@intel.com>,
	"kirill.shutemov@linux.intel.com"
	<kirill.shutemov@linux.intel.com>, "Christopherson,,
	Sean" <seanjc@google.com>,
	"Chatre, Reinette" <reinette.chatre@intel.com>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"Yamahata, Isaku" <isaku.yamahata@intel.com>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"Shahar, Sagi" <sagis@google.com>,
	"imammedo@redhat.com" <imammedo@redhat.com>,
	"Gao, Chao" <chao.gao@intel.com>,
	"Brown, Len" <len.brown@intel.com>,
	"sathyanarayanan.kuppuswamy@linux.intel.com" 
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	"Huang, Ying" <ying.huang@intel.com>,
	"Williams, Dan J" <dan.j.williams@intel.com>
Subject: Re: [PATCH v8 12/16] x86/virt/tdx: Designate the global KeyID and configure the TDX module
Date: Tue, 10 Jan 2023 08:25:42 -0800	[thread overview]
Message-ID: <755f94aa-a0cc-b7d6-ce8a-a81ff4f598da@intel.com> (raw)
In-Reply-To: <748adb0e8df5f804371f0587ed8fef1184177484.camel@intel.com>

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.