Re: [PATCH] tdx, memory hotplug: Check whole hot-adding memory range for TDX

[Huang, Ying]

Hi, David,

Thanks a lot for comments!

David Hildenbrand <david@xxxxxxxxxx> writes:

> 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 :)

Because only TDX compatible memory can be used for TDX guest.  The buddy
is used to allocate memory for TDX guest.  While devdax will not be used
for that.

>> This also makes the original TDX memory hotplug notifier useless, so
>> delete it.
>
> The online-notifier would even be too late when used with the
> memmap-on-memory feature I assume, as we might be touching that memory
> even before being able to call memory online notifiers.

This should be OK.  Because we will not use the memory for TDX guest in
this way.

> One way to handle that would be to switch to the MEM_PREPARE_ONLINE
> notifier, but it's still called per-memory block.
>
> Nothing jumped at me, so
>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thank you very much!

--
Best Regards,
Huang, Ying