Re: [RFC XEN PATCH v3 2/3] x86/pvh: Add (un)map_pirq and setup_gsi for PVH dom0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.12.2023 10:55, Roger Pau Monné wrote:
> On Thu, Dec 14, 2023 at 08:55:45AM +0000, Chen, Jiqian wrote:
>> On 2023/12/13 15:03, Jan Beulich wrote:
>>> On 13.12.2023 03:47, Chen, Jiqian wrote:
>>>> On 2023/12/12 17:30, Jan Beulich wrote:
>>>>> On 12.12.2023 07:49, Chen, Jiqian wrote:
>>>>>> On 2023/12/11 23:31, Roger Pau Monné wrote:
>>>>>>> On Mon, Dec 11, 2023 at 12:40:08AM +0800, Jiqian Chen wrote:
>>>>>>>> --- a/xen/arch/x86/hvm/hypercall.c
>>>>>>>> +++ b/xen/arch/x86/hvm/hypercall.c
>>>>>>>> @@ -72,8 +72,11 @@ long hvm_physdev_op(int cmd, 
>>>>>>>> XEN_GUEST_HANDLE_PARAM(void) arg)
>>>>>>>>  
>>>>>>>>      switch ( cmd )
>>>>>>>>      {
>>>>>>>> +    case PHYSDEVOP_setup_gsi:
>>>>>>>
>>>>>>> I think given the new approach on the Linux side patches, where
>>>>>>> pciback will configure the interrupt, there's no need to expose
>>>>>>> setup_gsi anymore?
>>>>>> The latest patch(the second patch of v3 on kernel side) does setup_gsi 
>>>>>> and map_pirq for passthrough device in pciback, so we need this and 
>>>>>> below.
>>>>>>
>>>>>>>
>>>>>>>>      case PHYSDEVOP_map_pirq:
>>>>>>>>      case PHYSDEVOP_unmap_pirq:
>>>>>>>> +        if ( is_hardware_domain(currd) )
>>>>>>>> +            break;
>>>>>>>
>>>>>>> Also Jan already pointed this out in v2: this hypercall needs to be
>>>>>>> limited so a PVH dom0 cannot execute it against itself.  IOW: refuse
>>>>>>> the hypercall if DOMID_SELF or the passed domid matches the current
>>>>>>> domain domid.
>>>>>> Yes, I remember Jan's suggestion, but since the latest patch(the second 
>>>>>> patch of v3 on kernel side) has change the implementation, it does 
>>>>>> setup_gsi and map_pirq for dom0 itself, so I didn't add the DOMID_SELF 
>>>>>> check.
>>>>>
>>>>> And why exactly would it do specifically the map_pirq? (Even the setup_gsi
>>>>> looks questionable to me, but there might be reasons there.)
>>>> Map_pirq is to solve the check failure problem. (pci_add_dm_done-> 
>>>> xc_domain_irq_permission-> XEN_DOMCTL_irq_permission-> 
>>>> pirq_access_permitted->domain_pirq_to_irq->return irq is 0)
>>>> Setup_gsi is because the gsi is never be unmasked, so the gsi is never be 
>>>> registered( vioapic_hwdom_map_gsi-> mp_register_gsi is never be called).
>>>
>>> And it was previously made pretty clear by Roger, I think, that doing a 
>>> "map"
>>> just for the purpose of granting permission is, well, at best a temporary
>>> workaround in the early development phase. If there's presently no hypercall
>>> to _only_ grant permission to IRQ, we need to add one.
>> Could you please describe it in detail? Do you mean to add a new hypercall 
>> to grant irq access for dom0 or domU?
>> It seems XEN_DOMCTL_irq_permission is the hypercall to grant irq access from 
>> dom0 to domU(see XEN_DOMCTL_irq_permission-> irq_permit_access). There is no 
>> need to add hypercall to grant irq access.
>> We failed here (XEN_DOMCTL_irq_permission-> 
>> pirq_access_permitted->domain_pirq_to_irq->return irq is 0) is because the 
>> PVH dom0 didn't use PIRQ, so we can't get irq from pirq if "current" is PVH 
>> dom0.
> 
> One way to bodge this would be to detect whether the caller of
> XEN_DOMCTL_irq_permission is a PV or an HVM domain, and in case of HVM
> assume the pirq field is a GSI.  I'm unsure however how that will work
> with non-x86 architectures.
> 
> It would  be better to introduce a new XEN_DOMCTL_gsi_permission, or
> maybe XEN_DOMCTL_intr_permission that can take a struct we can use to
> accommodate GSIs and other arch specific interrupt identifiers.

How would you see MSI being handled then?

Jan