Re: [XEN PATCH v10 4/5] tools: Add new function to get gsi from dev

["Chen, Jiqian" <Jiqian.Chen@xxxxxxx>]

On 2024/6/20 15:43, Jan Beulich wrote:
> On 20.06.2024 09:03, Chen, Jiqian wrote:
>> On 2024/6/18 17:13, Jan Beulich wrote:
>>> On 18.06.2024 10:10, Chen, Jiqian wrote:
>>>> On 2024/6/17 23:10, Jan Beulich wrote:
>>>>> On 17.06.2024 11:00, Jiqian Chen wrote:
>>>>>> --- a/tools/libs/light/libxl_pci.c
>>>>>> +++ b/tools/libs/light/libxl_pci.c
>>>>>> @@ -1406,6 +1406,12 @@ static bool pci_supp_legacy_irq(void)
>>>>>>  #endif
>>>>>>  }
>>>>>>  
>>>>>> +#define PCI_DEVID(bus, devfn)\
>>>>>> +            ((((uint16_t)(bus)) << 8) | ((devfn) & 0xff))
>>>>>> +
>>>>>> +#define PCI_SBDF(seg, bus, devfn) \
>>>>>> +            ((((uint32_t)(seg)) << 16) | (PCI_DEVID(bus, devfn)))
>>>>>
>>>>> I'm not a maintainer of this file; if I were, I'd ask that for 
>>>>> readability's
>>>>> sake all excess parentheses be dropped from these.
>>>> Isn't it a coding requirement to enclose each element in parentheses in 
>>>> the macro definition?
>>>> It seems other files also do this. See tools/libs/light/libxl_internal.h
>>>
>>> As said, I'm not a maintainer of this code. Yet while I'm aware that libxl
>>> has its own CODING_STYLE, I can't spot anything towards excessive use of
>>> parentheses there.
>> So, which parentheses do you think are excessive use?
> 
> #define PCI_DEVID(bus, devfn)\
>             (((uint16_t)(bus) << 8) | ((devfn) & 0xff))
> 
> #define PCI_SBDF(seg, bus, devfn) \
>             (((uint32_t)(seg) << 16) | PCI_DEVID(bus, devfn))
Thanks, will change in next version.

> 
>>>>>> @@ -1486,6 +1496,18 @@ static void pci_add_dm_done(libxl__egc *egc,
>>>>>>          goto out_no_irq;
>>>>>>      }
>>>>>>      if ((fscanf(f, "%u", &irq) == 1) && irq) {
>>>>>> +#ifdef CONFIG_X86
>>>>>> +        sbdf = PCI_SBDF(pci->domain, pci->bus,
>>>>>> +                        (PCI_DEVFN(pci->dev, pci->func)));
>>>>>> +        gsi = xc_physdev_gsi_from_dev(ctx->xch, sbdf);
>>>>>> +        /*
>>>>>> +         * Old kernel version may not support this function,
>>>>>
>>>>> Just kernel?
>>>> Yes, xc_physdev_gsi_from_dev depends on the function implemented on linux 
>>>> kernel side.
>>>
>>> Okay, and when the kernel supports it but the underlying hypervisor doesn't
>>> support what the kernel wants to use in order to fulfill the request, all
>> I don't know what things you mentioned hypervisor doesn't support are,
>> because xc_physdev_gsi_from_dev is to get the gsi of pcidev through sbdf 
>> information,
>> that relationship can be got only in dom0 instead of Xen hypervisor.
>>
>>> is fine? (See also below for what may be needed in the hypervisor, even if
>> You mean xc_physdev_map_pirq needs gsi?
> 
> I'd put it slightly differently: You arrange for that function to now take a
> GSI when the caller is PVH. But yes, the function, when used with
> MAP_PIRQ_TYPE_GSI, clearly expects a GSI as input (see also below).
> 
>>> this IOCTL would be satisfied by the kernel without needing to interact with
>>> the hypervisor.)
>>>
>>>>>> +         * so if fail, keep using irq; if success, use gsi
>>>>>> +         */
>>>>>> +        if (gsi > 0) {
>>>>>> +            irq = gsi;
>>>>>
>>>>> I'm still puzzled by this, when by now I think we've sufficiently 
>>>>> clarified
>>>>> that IRQs and GSIs use two distinct numbering spaces.
>>>>>
>>>>> Also, as previously indicated, you call this for PV Dom0 as well. Aiui on
>>>>> the assumption that it'll fail. What if we decide to make the 
>>>>> functionality
>>>>> available there, too (if only for informational purposes, or for
>>>>> consistency)? Suddenly you're fallback logic wouldn't work anymore, and
>>>>> you'd call ...
>>>>>
>>>>>> +        }
>>>>>> +#endif
>>>>>>          r = xc_physdev_map_pirq(ctx->xch, domid, irq, &irq);
>>>>>
>>>>> ... the function with a GSI when a pIRQ is meant. Imo, as suggested 
>>>>> before,
>>>>> you strictly want to avoid the call on PV Dom0.
>>>>>
>>>>> Also for PVH Dom0: I don't think I've seen changes to the hypercall
>>>>> handling, yet. How can that be when GSI and IRQ aren't the same, and hence
>>>>> incoming GSI would need translating to IRQ somewhere? I can once again 
>>>>> only
>>>>> assume all your testing was done with IRQs whose numbers happened to match
>>>>> their GSI numbers. (The difference, imo, would also need calling out in 
>>>>> the
>>>>> public header, where the respective interface struct(s) is/are defined.)
>>>> I feel like you missed out on many of the previous discussions.
>>>> Without my changes, the original codes use irq (read from file 
>>>> /sys/bus/pci/devices/<sbdf>/irq) to do xc_physdev_map_pirq,
>>>> but xc_physdev_map_pirq require passing into gsi instead of irq, so we 
>>>> need to use gsi whether dom0 is PV or PVH, so for the original codes, they 
>>>> are wrong.
>>>> Just because by chance, the irq value in the Linux kernel of pv dom0 is 
>>>> equal to the gsi value, so there was no problem with the original pv 
>>>> passthrough.
>>>> But not when using PVH, so passthrough failed.
>>>> With my changes, I got gsi through function xc_physdev_gsi_from_dev 
>>>> firstly, and to be compatible with old kernel versions(if the ioctl
>>>> IOCTL_PRIVCMD_GSI_FROM_DEV is not implemented), I still need to use the 
>>>> irq number, so I need to check the result
>>>> of gsi, if gsi > 0 means IOCTL_PRIVCMD_GSI_FROM_DEV is implemented I can 
>>>> use the right one gsi, otherwise keep using wrong one irq. 
>>>
>>> I understand all of this, to a (I think) sufficient degree at least. Yet 
>>> what
>>> you're effectively proposing (without explicitly saying so) is that e.g.
>>> struct physdev_map_pirq's pirq field suddenly may no longer hold a pIRQ
>>> number, but (when the caller is PVH) a GSI. This may be a necessary 
>>> adjustment
>>> to make (simply because the caller may have no way to express things in pIRQ
>>> terms), but then suitable adjustments to the handling of PHYSDEVOP_map_pirq
>>> would be necessary. In fact that field is presently marked as "IN or OUT";
>>> when re-purposed to take a GSI on input, it may end up being necessary to 
>>> pass
>>> back the pIRQ (in the subject domain's numbering space). Or alternatively it
>>> may be necessary to add yet another sub-function so the GSI can be 
>>> translated
>>> to the corresponding pIRQ for the domain that's going to use the IRQ, for 
>>> that
>>> then to be passed into PHYSDEVOP_map_pirq.
>> If I understood correctly, your concerns about this patch are two:
>> First, when dom0 is PV, I should not use xc_physdev_gsi_from_dev to get gsi 
>> to do xc_physdev_map_pirq, I should keep the original code that use irq.
> 
> Yes.
OK, I can change to do this.
But I still have a concern:
Although without my changes, passthrough can work now when dom0 is PV.
And you also agree that: for xc_physdev_map_pirq, when use with 
MAP_PIRQ_TYPE_GSI, it expects a GSI as input.
Isn't it a wrong for PV dom0 to pass irq in? Why don't we use gsi as it should 
be used, since we have a function to get gsi now?

> 
>> Second, when dom0 is PVH, I get the gsi, but I should not pass gsi as the 
>> fourth parameter of xc_physdev_map_pirq, I should create a new local 
>> parameter pirq=-1, and pass it in.
> 
> I think so, yes. You also may need to record the output value, so you can 
> later
> use it for unmap. xc_physdev_map_pirq() may also need adjusting, as right now
> it wouldn't put a negative incoming *pirq into the .pirq field. 
xc_physdev_map_pirq's logic is if we pass a negative in, it sets *pirq to 
index(gsi).
Is its logic right? If not how do we change it?

> I actually wonder if that's even correct right now, i.e. independent of your 
> change.
Even without my changes, passthrough can work for PV dom0, not for PVH dom0.
According to the logic of hypercall PHYSDEVOP_map_pirq,
if pirq is -1, it calls physdev_map_pirq-> allocate_and_map_gsi_pirq-> 
allocate_pirq -> get_free_pirq to get pirq.
If pirq is set to positive before calling hypercall, it set pirq to its own 
value in allocate_pirq.

> 
> Jan

-- 
Best regards,
Jiqian Chen.