Re: [PATCH v9 10/13] cxl: Enable AMD Zen5 address translation using ACPI PRMT

[Dave Jiang <dave.jiang@intel.com>]

On 1/19/26 7:33 AM, Robert Richter wrote:
> (+Rafael and some AMD folks)
> 
> Hi Peter,
> 
> On Fri, Jan 16, 2026 at 03:38:38PM +0100, Peter Zijlstra wrote:
>> On Thu, Jan 15, 2026 at 09:30:10AM +0100, Ard Biesheuvel wrote:
>>> On Thu, 15 Jan 2026 at 09:04, Peter Zijlstra <peterz@infradead.org> wrote:
>>>>
>>>> On Wed, Jan 14, 2026 at 06:08:59PM +0000, Jonathan Cameron wrote:
>>>>
>>>>> Do we have a potential issue wrt to merging this as it stands and improving
>>>>> on it later?  i.e. Is this a blocking issue for this patch set?
>>>>
>>>> Well, why do you *have* to use PRMT at all? And this is a serious
>>>> question; PRMT is basically injecting unaudited magic code into the
>>>> kernel, and that is a security risk.
>>>>
>>>> Worse, in order to run this shit, we have to lower or disable various
>>>> security measures.
>>>>
>>>
>>> Only if we decide to keep running it privileged, which the PRM spec no
>>> longer requires (as you have confirmed yourself when we last discussed
>>> this, right?)
>>
>> Indeed. But those very constraints also make me wonder why we would ever
>> bother with PRM at all, and not simply require a native driver. Then you
>> actually *know* what the thing does and can debug/fix it without having
>> to rely on BIOS updates and whatnot.
> 
> an address translation driver needs the configuration data from the
> Data Fabric, which is only known to firmware but not to the kernel.
> Other ways would be necessary to expose and calculate that data, if it
> is even feasible to make this information available.
> 
> So using PRM looks reasonable to me as this abstracts the logic and
> data behind a method, same as doing a library call. Of course, you
> don't want to trust that, but that could be addressed running it
> unprivileged.
> 
>> Worse, you might have to deal with various incompatible buggy PRM
>> versions because BIOS :/
> 
> The address translation functions are straight forward. I haven't
> experienced any issues here. If there would be any, this will be
> solvable, e.g. by requiring a specific minimum version or uuid to run
> PRM.
> 
>>
>>>> If I had my way, we would WARN and TAINT the kernel whenever such
>>>> garbage got used.
>>>
>>> These are things that used to live in SMM, requiring all CPUs to
>>> disappear into SMM mode in a way that was completely opaque to the OS.
>>>
>>> PRM runs under the control of the OS, does not require privileges and
>>> only needs MMIO access to the regions it describes in its manifest
>>> (which the OS can inspect, if desired). So if there are security
>>> concerns with PRM today, it is because we were lazy and did not
>>> implement PRM securely from the beginning.
>>>
>>> In my defense, I wasn't aware of the unprivileged requirement until
>>> you spotted it recently: it was something I had asked for when the PRM
>>> spec was put up for "review" by the Intel and MS authors, and they
>>> told me they couldn't possibly make any changes at that point, because
>>> it had already gone into production. But as it turns out, the change
>>> was made after all.
>>>
>>> I am a total noob when it comes to how x86 does its ring0/ring3
>>> switching, but with some help, I should be able to prototype something
>>> to call into the PRM service unprivileged, running under the efi_mm.
>>
>> The ring transition itself is done using IRET; create a iret frame with
>> userspace CS and the right IP (and flag etc.) and off you go. The
>> problem is getting back in the kernel I suppose. All the 'normal' kernel
>> entry points assume the kernel stack is empty and all that.
>>
>> The whole usermodehelper stuff creates a whole extra thread, sets
>> everything up and drops into userspace. Perhaps that is the easiest
>> solution. Basically you set the thread's mm to efi_mm, populate
>> task_pt_regs() with the right bits and simply drop into 'userspace'.
>>
>> Then it can complete by terminating itself (sys_exit()) and the calling
>> context reaps the thing and continues.
> 
> I can help with testing and also work on securing the PRM calls.
> Thanks Ard for also looking into this.
> 
>>
>>> Would that allay your concerns?
>>
>> Yeah, running it as userspace would be fine; we don't trust that.
>>
>> But again; a native driver is ever so much better than relying on PRM.
>>
>> In this case it is AMD doing a driver for their own chips, they know how
>> they work, they should be able to write this natively.
> 
> Since a native driver introduces additional issues, as explained
> above, I would prefer to use PRM for address translation and instead
> ensure the PRM call is secure.
> 
> Dan, Dave, regarding this series, the cxl driver just uses existing
> PRM kernel code and does not implement anything new here. Is there
> anything that would prevent this series from being accepted? We are
> already at v10 and review is complete:
> 
> https://patchwork.kernel.org/project/cxl/list/?series=1042412
> 
> I will follow up with working on unprivileged PRM calls. I think, that
> will be the best solution here.

I have no objections with the promise of work on unprivileged PRM call. Please rev the convention doc with Dan's request and we can get this merged.

> 
> Thanks,
> 
> -Robert