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

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Message-ID: <cf4a3ae4-deae-4224-88e3-308a55492085@redhat.com>
Date: Mon, 30 Sep 2024 10:58:09 +0200
From: David Hildenbrand <david@...hat.com>
To: 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,
 Dan Williams <dan.j.williams@...el.com>, 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 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 :)

> 
> 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.

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@...hat.com>

-- 
Cheers,

David / dhildenb