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

[Jan Beulich <jbeulich@xxxxxxxx>]

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))

>>>>> @@ -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.

> 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. I actually
wonder if that's even correct right now, i.e. independent of your change.

Jan