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

[Jan Beulich <jbeulich@xxxxxxxx>]

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".

Personally I'm taking what's written in a pretty strict sense: If in
doubt, drop tags which may no longer cover all changes they would
apply to. (This is, in my interpretation, generally less of a problem
for A-b, as that only conveys "this kind of change is okay to make",
without covering much of the details. In the case here retaining A-b
would probably have been acceptable, albeit there's still room for
interpretation. For example, if an A-b was offered based on somebody
else's R-b, then likely the A-b would need dropping if the R-b is
dropped.)

Jan