linux-kernel - Re: [PATCH v8 07/16] x86/virt/tdx: Use all system memory when initializing TDX module as TDX memory

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Message-ID: <16f23950-2a27-29de-c0b4-e5f2d927c8b4@intel.com>
Date:   Tue, 10 Jan 2023 08:18:46 -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>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "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 07/16] x86/virt/tdx: Use all system memory when
 initializing TDX module as TDX memory

On 1/10/23 04:09, Huang, Kai wrote:
> On Mon, 2023-01-09 at 08:51 -0800, Dave Hansen wrote:
>> On 1/9/23 03:48, Huang, Kai wrote:
>>>>>>> This can also be enhanced in the future, i.e. by allowing adding non-TDX
>>>>>>> memory to a separate NUMA node.  In this case, the "TDX-capable" nodes
>>>>>>> and the "non-TDX-capable" nodes can co-exist, but the kernel/userspace
>>>>>>> needs to guarantee memory pages for TDX guests are always allocated from
>>>>>>> the "TDX-capable" nodes.
>>>>>
>>>>> Why does it need to be enhanced?  What's the problem?
>>>
>>> The problem is after TDX module initialization, no more memory can be hot-added
>>> to the page allocator.
>>>
>>> Kirill suggested this may not be ideal. With the existing NUMA ABIs we can
>>> actually have both TDX-capable and non-TDX-capable NUMA nodes online. We can
>>> bind TDX workloads to TDX-capable nodes while other non-TDX workloads can
>>> utilize all memory.
>>>
>>> But probably it is not necessarily to call out in the changelog?
>>
>> Let's say that we add this TDX-compatible-node ABI in the future.  What
>> will old code do that doesn't know about this ABI?
> 
> Right.  The old app will break w/o knowing the new ABI.  One resolution, I
> think, is we don't introduce new userspace ABI, but hide "TDX-capable" and "non-
> TDX-capable" nodes in the kernel, and let kernel to enforce always allocating
> TDX guest memory from those "TDX-capable" nodes.

That doesn't actually hide all of the behavior from users.  Let's say
they do:

	numactl --membind=6 qemu-kvm ...

In other words, take all of this guest's memory and put it on node 6.
There lots of free memory on node 6 which is TDX-*IN*compatible.  Then,
they make it a TDX guest:

	numactl --membind=6 qemu-kvm -tdx ...

What happens?  Does the kernel silently ignore the --membind=6?  Or does
it return -ENOMEM somewhere and confuse the user who has *LOTS* of free
memory on node 6.

In other words, I don't think the kernel can just enforce this
internally and hide it from userspace.

>> Is there something fundamental that keeps a memory area that spans two
>> nodes from being removed and then a new area added that is comprised of
>> a single node?
>> Boot time:
>>
>> 	| memblock  |  memblock |
>> 	<--Node=0--> <--Node=1-->
>>
>> Funky hotplug... nothing to see here, then:
>>
>> 	<--------Node=2-------->
> 
> I must have missed something, but how can this happen?
> 
> I had memory that this cannot happen because the BIOS always allocates address
> ranges for all NUMA nodes during machine boot.  Those address ranges don't
> necessarily need to have DIMM fully populated but they don't change during
> machine's runtime.

Is your memory correct?  Is there evidence, or requirements in any
specification to support your memory?