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 09:55, 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.

Hmm, yes and no. May I turn your attention to
https://lists.xen.org/archives/html/xen-devel/2023-07/msg02056.html
and its earlier version
https://lists.xen.org/archives/html/xen-devel/2023-05/msg00301.html
(it's imo a shame that this series continues to be stuck)?

Both make pretty clear that without pIRQ, this domctl cannot be used in
its present shape anyway, for ...

> 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.

... this very reason. Addressing this one way or another is a necessary
part of making passthrough work with PVH Dom0. So _effectively_ there is
no hypercall allowing PVH Dom0 to grant IRQ permission.

> So, it seems the logic of XEN_DOMCTL_irq_permission is not suitable when PVH 
> dom0?

That's my view, yes.

> Maybe it directly needs to get irq from the caller(domU) instead of "current" 
> if the "current" has no PIRQ flag?

I don't think the IRQ mapping in the DomU is necessary to be known here.
What we want to grant is access to a host resource. That host resource is
therefore all that should need specifying for the operation to be carried
out. It just so happens that a PV Dom0 would specify the host IRQ by way
of supplying its own equivalent pIRQ.

Things are more "interesting" for MSI, though: The (Xen) IRQ may not be
known early enough. There wants to be a way of indicating that when such
an IRQ is created, permission should be granted to the domain that is
going to use that IRQ (by way of being assigned the respective device).
(This aspect may be part of why "map" presently also grants permission,
yet I continue to think that was wrong from the start. The more that
access there is [likely needlessly] granted to the domain requesting the
mapping, just for it to then further grant access to the DomU.)

Jan