Re: [RFC PATCH v4 5/8] xen/domctl: extend XEN_DOMCTL_assign_device to handle not only iommu

[Oleksii Moisieiev <Oleksii_Moisieiev@xxxxxxxx>]

Adding Paul to the conversation.


On 23/06/2025 10:15, Jan Beulich wrote:
> On 19.06.2025 18:15, Oleksii Moisieiev wrote:
>> On 18/06/2025 03:04, Stefano Stabellini wrote:
>>> On Thu, 12 Jun 2025, Oleksii Moisieiev wrote:
>>>> Hi Stefano,
>>>>
>>>> I'm very sorry for a long silence. Please see my answers below:
>>>>
>>>> On 22/05/2025 03:25, Stefano Stabellini wrote:
>>>>> On Mon, 19 May 2025, Oleksii Moisieiev wrote:
>>>>>> From: Grygorii Strashko<grygorii_strashko@xxxxxxxx>
>>>>>>
>>>>>> Add chained handling of assigned DT devices to support access-controller
>>>>>> functionality through SCI framework, so DT device assign request can be
>>>>>> passed to FW for processing and enabling VM access to requested device
>>>>>> (for example, device power management through FW interface like SCMI).
>>>>>>
>>>>>> The SCI access-controller DT device processing is chained after IOMMU
>>>>>> processing and expected to be executed for any DT device regardless of 
>>>>>> its
>>>>>> protection by IOMMU (or if IOMMU is disabled).
>>>>>>
>>>>>> This allows to pass not only IOMMU protected DT device through
>>>>>> xl.cfg:"dtdev" property for processing:
>>>>>>
>>>>>> dtdev = [
>>>>>>        "/soc/video@e6ef0000", <- IOMMU protected device
>>>>>>        "/soc/i2c@e6508000", <- not IOMMU protected device
>>>>>> ]
>>>>>>
>>>>>> The change is done in two parts:
>>>>>> 1) update iommu_do_dt_domctl() to check for dt_device_is_protected() and
>>>>>> not fail if DT device is not protected by IOMMU
>>>>>> 2) add chained call to sci_do_domctl() in do_domctl()
>>>>>>
>>>>>> Signed-off-by: Grygorii Strashko<grygorii_strashko@xxxxxxxx>
>>>>>> Signed-off-by: Oleksii Moisieiev<oleksii_moisieiev@xxxxxxxx>
>>>>>> ---
>>>>>>
>>>>>>
>>>>>>
>>>>>>     xen/arch/arm/firmware/sci.c             | 37 
>>>>>> +++++++++++++++++++++++++
>>>>>>     xen/arch/arm/include/asm/firmware/sci.h | 14 ++++++++++
>>>>>>     xen/common/domctl.c                     | 19 +++++++++++++
>>>>>>     xen/drivers/passthrough/device_tree.c   |  6 ++++
>>>>>>     4 files changed, 76 insertions(+)
>>>>>>
>>>>>> diff --git a/xen/arch/arm/firmware/sci.c b/xen/arch/arm/firmware/sci.c
>>>>>> index e1522e10e2..8efd541c4f 100644
>>>>>> --- a/xen/arch/arm/firmware/sci.c
>>>>>> +++ b/xen/arch/arm/firmware/sci.c
>>>>>> @@ -126,6 +126,43 @@ int sci_assign_dt_device(struct domain *d, struct 
>>>>>> dt_device_node *dev)
>>>>>>         return 0;
>>>>>>     }
>>>>>>     
>>>>>> +int sci_do_domctl(struct xen_domctl *domctl, struct domain *d,
>>>>>> +                  XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
>>>>>> +{
>>>>>> +    struct dt_device_node *dev;
>>>>>> +    int ret = 0;
>>>>>> +
>>>>>> +    switch ( domctl->cmd )
>>>>>> +    {
>>>>>> +    case XEN_DOMCTL_assign_device:
>>>>>> +        ret = -EOPNOTSUPP;
>>>>> Are you sure -EOPNOTSUPP is the right error code for the 3 checks below?
>>>> The -EOPNOTSUPP code is used because this is part of a chained call after
>>>> iommu_do_domctl, as stated in xen/common/domctl.c:859. The
>>>> XEN_DOMCTL_assign_device
>>>> call is expected to handle any DT device, regardless of whether the DT
>>>> device is
>>>> protected by an IOMMU or if the IOMMU is disabled.
>>>> The following cases are considered:
>>>>
>>>> 1. IOMMU Protected Device (Success)
>>>>
>>>> If the device is protected by the IOMMU and iommu_do_domctl returns 0,
>>>> we continue
>>>> processing the DT device by calling sci_do_domctl.
>>>>
>>>> 2. IOMMU Disabled (-EOPNOTSUPP from iommu_do_domctl)
>>>>
>>>> If iommu_do_domctl returns -EOPNOTSUPP, indicating that the IOMMU is
>>>> disabled,
>>>> we still proceed to call sci_do_domctl.
>>> OK this makes sense.  I think it is OK to have a special error code to
>>> say "the IOMMU is disabled" but I don't know if it is a good idea to try
>>> to use -EOPNOTSUPP for that. -EOPNOTSUPP could mean a hypervisor
>>> configuration with domctl disabled, for instance.
>>>
>>> It might be wiser to use a different error code. Maybe ENOENT?
>>>
>> I see that in the following commit:
>>
>> 71e617a6b8 (use is_iommu_enabled() where appropriate..., 2019-09-17)
>>
>> -ENOSYS return code was changed to -EOPNOTSUPP in iommu_do_domctl.
>>
>> It's not clear to me why this was done from the commit description.
> This has been discussed many times elsewhere. Many of our ENOSYS uses are
> simply wrong. ENOSYS has very limited applicability: Unavailability of a
> top-level hypercall (originally: syscall).
>
>> Maybe we should add commit author?
> You might, but Paul hasn't been active in Xen for quite some time now.
>
> Jan