Message-ID: <c9dcdc44-7031-4541-96a2-70a071accb61@redhat.com>
Date: Tue, 1 Oct 2024 09:56:29 +0200
From: David Hildenbrand <david@...hat.com>
To: Dan Williams <dan.j.williams@...el.com>, Huang Ying
 <ying.huang@...el.com>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: x86@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
 Oscar Salvador <osalvador@...e.de>, linux-coco@...ts.linux.dev,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Kai Huang <kai.huang@...el.com>, "H. Peter Anvin" <hpa@...or.com>,
 Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] tdx, memory hotplug: Check whole hot-adding memory range
 for TDX

On 01.10.24 08:45, Dan Williams wrote:
> David Hildenbrand wrote:
>> On 30.09.24 07:51, Huang Ying wrote:
>>> On systems with TDX (Trust Domain eXtensions) enabled, memory ranges
>>> hot-added must be checked for compatibility by TDX.  This is currently
>>> implemented through memory hotplug notifiers for each memory_block.
>>> If a memory range which isn't TDX compatible is hot-added, for
>>> example, some CXL memory, the command line as follows,
>>>
>>>     $ echo 1 > /sys/devices/system/node/nodeX/memoryY/online
>>>
>>> will report something like,
>>>
>>>     bash: echo: write error: Operation not permitted
>>>
>>> If pr_debug() is enabled, the error message like below will be shown
>>> in the kernel log,
>>>
>>>     online_pages [mem 0xXXXXXXXXXX-0xXXXXXXXXXX] failed
>>>
>>> Both are too general to root cause the problem.  This will confuse
>>> users.  One solution is to print some error messages in the TDX memory
>>> hotplug notifier.  However, memory hotplug notifiers are called for
>>> each memory block, so this may lead to a large volume of messages in
>>> the kernel log if a large number of memory blocks are onlined with a
>>> script or automatically.  For example, the typical size of memory
>>> block is 128MB on x86_64, when online 64GB CXL memory, 512 messages
>>> will be logged.
>>
>> ratelimiting would likely help here a lot, but I agree that it is
>> suboptimal.
>>
>>>
>>> Therefore, in this patch, the whole hot-adding memory range is checked
>>> for TDX compatibility through a newly added architecture specific
>>> function (arch_check_hotplug_memory_range()).  If rejected, the memory
>>> hot-adding will be aborted with a proper kernel log message.  Which
>>> looks like something as below,
>>>
>>>     virt/tdx: Reject hot-adding memory range: 0xXXXXXXXX-0xXXXXXXXX for TDX compatibility.
>>   > > The target use case is to support CXL memory on TDX enabled systems.
>>> If the CXL memory isn't compatible with TDX, the whole CXL memory
>>> range hot-adding will be rejected.  While the CXL memory can still be
>>> used via devdax interface.
>>
>> I'm curious, why can that memory be used through devdax but not through
>> the buddy? I'm probably missing something important :)
> 
> TDX requires memory that supports integrity and encryption. Until
> platforms and expanders with a technology called CXL TSP arrives, CXL
> memory is not able to join the TCB.
> 
> The TDX code for simplicity assumes that only memory present at boot
> might be capable of TDX and that everything else is not.

So is there ever a chance where add_memory() would actually work now 
with TDX? Or can we just simplify and unconditionally reject 
add_memory() if TDX is enabled?

> 
> Confidential VMs use guest_mem_fd to allocate memory, and that only
> pulls from the page allocator as a backend.
> 
> This ability to use devdax in an offline mode is a hack to not

Thanks, I was missing the "hack" of it, and somehow (once again) assumed 
that we would be hotplugging memory into confidential VMs.

-- 
Cheers,

David / dhildenb

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