Re: [PATCH v3] xen/arm: vpsci: ignore upper 32 bits for SMC32 PSCI arguments

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.04.2026 11:51, Mykola Kvach wrote:
> On Wed, Apr 1, 2026 at 12:22 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> On 01.04.2026 10:49, Mykola Kvach wrote:
>>> On Wed, Apr 1, 2026 at 11:14 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>> On 01.04.2026 09:13, Mykola Kvach wrote:
>>>>> On Wed, Apr 1, 2026 at 9:29 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>>> On 31.03.2026 20:31, Mykola Kvach wrote:
>>>>>>> From: Mykola Kvach <mykola_kvach@xxxxxxxx>
>>>>>>>
>>>>>>> SMCCC DEN0028G, section 3.1, states that for AArch64 SMC/HVC calls
>>>>>>> using Wn, only the least significant 32 bits are significant and the
>>>>>>> upper 32 bits must be ignored by the implementation.
>>>>>>>
>>>>>>> So for SMC32 PSCI calls, Xen must not treat non-zero upper bits in the
>>>>>>> argument registers as an error. Instead, they should be discarded when
>>>>>>> decoding the arguments.
>>>>>>>
>>>>>>> Arm ARM DDI 0487J.a (D1-5406) also notes that the upper 32 bits may be
>>>>>>> implementation defined when entering from AArch32. Xen zeros them on
>>>>>>> entry, but that guarantee is only relevant for 32-bit domains.
>>>>>>>
>>>>>>> Update PSCI v0.2+ CPU_ON, CPU_SUSPEND, AFFINITY_INFO and SYSTEM_SUSPEND
>>>>>>> to read SMC32 arguments via PSCI_ARG32(), while keeping the SMC64
>>>>>>> handling unchanged.
>>>>>>>
>>>>>>> No functional change is intended for PSCI 0.1.
>>>>>>>
>>>>>>> Suggested-by: Julien Grall <julien@xxxxxxx>
>>>>>>> Signed-off-by: Mykola Kvach <mykola_kvach@xxxxxxxx>
>>>>>>> Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
>>>>>>
>>>>>> I thought I might as well include this in my next commit sweep, but isn't
>>>>>> this R-b being invalidated by ...
>>>>>>
>>>>>>> ---
>>>>>>> v3:
>>>>>>>  - use PSCI_ARG_CONV for SYSTEM_SUSPEND
>>>>>>
>>>>>> ... this change. That's ...
>>>>>>
>>>>>>> @@ -422,14 +427,8 @@ bool do_vpsci_0_2_call(struct cpu_user_regs *regs, 
>>>>>>> uint32_t fid)
>>>>>>>      case PSCI_1_0_FN32_SYSTEM_SUSPEND:
>>>>>>>      case PSCI_1_0_FN64_SYSTEM_SUSPEND:
>>>>>>>      {
>>>>>>> -        register_t epoint = PSCI_ARG(regs, 1);
>>>>>>> -        register_t cid = PSCI_ARG(regs, 2);
>>>>>>> -
>>>>>>> -        if ( fid == PSCI_1_0_FN32_SYSTEM_SUSPEND )
>>>>>>> -        {
>>>>>>> -            epoint &= GENMASK(31, 0);
>>>>>>> -            cid &= GENMASK(31, 0);
>>>>>>> -        }
>>>>>>> +        register_t epoint = PSCI_ARG_CONV(regs, 1, is_conv_64);
>>>>>>> +        register_t cid = PSCI_ARG_CONV(regs, 2, is_conv_64);
>>>>>>>
>>>>>>>          perfc_incr(vpsci_system_suspend);
>>>>>>>          PSCI_SET_RESULT(regs, do_psci_1_0_system_suspend(epoint, cid));
>>>>>>
>>>>>> ... this hunk aiui, which is far from merely cosmetic imo. While
>>>>>
>>>>> Nobody said that the change had to be purely cosmetic in order to keep
>>>>> the tag. I understood it differently from the official Xen
>>>>> documentation pages.
>>>>>
>>>>>> behavior looks to remain the same for PSCI_1_0_FN32_SYSTEM_SUSPEND, it
>>>>>
>>>>> Exactly. If the changes are not substantial, I do not see a reason to
>>>>> drop the tag ...
>>>>>
>>>>>> clearly changes for PSCI_1_0_FN64_SYSTEM_SUSPEND. That may be intended
>>>>>> and for the better, but the change clearly wasn't reviewed by Bertrand,
>>>>>> nor - when offering the R-b - did he ask for this extra change.
>>>>>
>>>>> ... and this is also how I understood the Xen patch submission
>>>>> guidelines [1], which say:
>>>>>
>>>>> "Note that if there are several revisions of a patch, you ought to
>>>>> copy tags that have accumulated during the review. For example, if
>>>>> person A and person B added a Reviewed-by: tag to v1 of your patch,
>>>>> include it into v2 of your patch. If you make substantial changes
>>>>> after certain tags were already applied, you will want to consider
>>>>> which ones are no longer applicable (and may require re-providing)."
>>>>>
>>>>> So my understanding was that tags should normally be kept across
>>>>> revisions, unless the changes are substantial enough to make them no
>>>>> longer applicable.
>>>>
>>>> Maybe our understanding of "substantial" differs. To me that's anything
>>>> changing functionality. Style adjustments, typo corrections, and alike
>>>> generally aren't substantial (albeit even then there may be exceptions).
>>>
>>> Thanks for clarifying what you consider substantial.
>>>
>>> Even under that interpretation, I do not see a functionality change
>>> here. "Refactoring" seems like the more accurate term in this case:
>>> the internal form changes, but the intended external behavior does
>>> not.
>>>
>>> It may be that we are using "functional change" in slightly different
>>> senses here.
>>>
>>> For v3, the switch to PSCI_ARG_CONV() in SYSTEM_SUSPEND was meant to
>>> make this case consistent with the helper-based argument decoding used
>>> elsewhere, not to change behavior.
>>>
>>> In particular, I do not see a functional change for
>>> PSCI_1_0_FN64_SYSTEM_SUSPEND: v2 used PSCI_ARG(regs, 1/2), and in v3
>>> PSCI_ARG_CONV(regs, 1/2, is_conv_64) should resolve to the same thing
>>> when is_conv_64 is true.
>>
>> Isn't the whole point of the patch to alter behavior when is_conv_64 is
>> false? For that case PSCI_1_0_FN64_SYSTEM_SUSPEND behavior looks to
>> change in v3, when it didn't in v2. Whereas for
>> PSCI_1_0_FN32_SYSTEM_SUSPEND the v3 change indeed only eliminates open-
>> coding, which one may or may not regard as "substantial".
> 
> I think the point I was trying to make is slightly narrower: in this
> code path, is_conv_64 is derived directly from fid via
> smccc_is_conv_64(fid) before the switch (fid).
> 
> So for PSCI_1_0_FN64_SYSTEM_SUSPEND, I do not see how
> is_conv_64 == false could arise here: if we are in the FN64 case,
> the function ID already encodes the 64-bit convention.
> 
> Conversely, if is_conv_64 is false here, then this cannot be the
> FN64 case.

Ah, I see. To figure that out, I would have had to do a proper review. I
was after committing only, which ought to be an entirely mechanical step.

> On that basis, I do not see a behavioral change for the FN64
> SYSTEM_SUSPEND case in v3.

I agree (now). I'm still not going to pick up that patch, but rather
leave it to the Arm maintainers. While not as clear cut as it first
seemed to me, I still consider it within the grey area.

Jan