Re: [PATCH v4] hw/pci: enforce use of slot only slot 0 when devices have an upstream PCIE port

[Ani Sinha <anisinha@redhat.com>]

> On 20-Jun-2023, at 4:13 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> 
> On Tue, Jun 20, 2023 at 12:48:05PM +0530, Ani Sinha wrote:
>> When a device has an upstream PCIE port, we can only use slot 0.
> 
> Actually, it's when device is plugged into a PCIE port.
> So maybe:
> 
> 	PCI Express ports only have one slot, so
> 	PCI Express devices can only be plugged into
> 	slot 0 on a PCIE port
> 
>> Non-zero slots
>> are invalid. This change ensures that we throw an error if the user
>> tries to hotplug a device with an upstream PCIE port to a non-zero slot.
> 
> it also adds a comment explaining why function 0 must not exist
> when function != 0 is added. or maybe split that part out.
> 
>> CC: jusual@redhat.com
>> CC: imammedo@redhat.com
>> Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2128929
>> Signed-off-by: Ani Sinha <anisinha@redhat.com>
>> ---
>> hw/pci/pci.c | 18 ++++++++++++++++++
>> 1 file changed, 18 insertions(+)
>> 
>> changelog:
>> v2: addressed issue with multifunction pcie root ports. Should allow
>> hotplug on functions other than function 0.
>> v3: improved commit message.
>> v4: improve commit message and code comments further. Some more
>> improvements might come in v5. No claims made here that this is
>> the final one :-)
>> 
>> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
>> index bf38905b7d..30ce6a78cb 100644
>> --- a/hw/pci/pci.c
>> +++ b/hw/pci/pci.c
>> @@ -64,6 +64,7 @@ bool pci_available = true;
>> static char *pcibus_get_dev_path(DeviceState *dev);
>> static char *pcibus_get_fw_dev_path(DeviceState *dev);
>> static void pcibus_reset(BusState *qbus);
>> +static bool pcie_has_upstream_port(PCIDevice *dev);
>> 
>> static Property pci_props[] = {
>>     DEFINE_PROP_PCI_DEVFN("addr", PCIDevice, devfn, -1),
>> @@ -1182,6 +1183,11 @@ static PCIDevice *do_pci_register_device(PCIDevice *pci_dev,
>>     } else if (dev->hotplugged &&
>>                !pci_is_vf(pci_dev) &&
>>                pci_get_function_0(pci_dev)) {
>> +        /*
>> +         * populating function 0 triggers a bus scan from the guest that
>> +         * exposes other non-zero functions. Hence we need to ensure that
>> +         * function 0 wasn't added yet.
>> +         */
> 
> Pls capitalize populating. Also, comments like this should come
> before the logic they document, not after. By the way it doesn't
> have to be a bus scan - I'd just say "a scan" - with ACPI
> guest knows what was added and can just probe the device functions.
> 
>>         error_setg(errp, "PCI: slot %d function 0 already occupied by %s,"
>>                    " new func %s cannot be exposed to guest.",
>>                    PCI_SLOT(pci_get_function_0(pci_dev)->devfn),
>> @@ -1189,6 +1195,18 @@ static PCIDevice *do_pci_register_device(PCIDevice *pci_dev,
>>                    name);
>> 
>>        return NULL;
>> +    } else if (dev->hotplugged &&
> 
> why hotplugged? Doesn't the same rule apply to all devices?
> 
>> +               !pci_is_vf(pci_dev) &&
> 
> Hmm. I think you copied it from here:
>    } else if (dev->hotplugged &&
>               !pci_is_vf(pci_dev) &&
>               pci_get_function_0(pci_dev)) {
> 
> it makes sense there because VFs are added later
> after PF exists.

I thought PFs are handled only in the host OS and only VFs are passthrough into the guest? I thought this check was because VFs have a different domain address separate from other non-vf devices in the guest PCI tree. 

> 
> But here it makes no sense that I can see.
> 
> 
>> +               pcie_has_upstream_port(pci_dev) && PCI_SLOT(devfn)) {
>> +        /*
>> +         * If the device has an upstream PCIE port, like a pcie root port,
> 
> no, a root port can not be an upstream port.
> 
> 
>> +         * we only support functions on slot 0.
>> +         */
>> +        error_setg(errp, "PCI: slot %d is not valid for %s,"
>> +                   " only functions on slot 0 is supported for devices"
>> +                   " with an upstream PCIE port.",
> 
> 
> something like:
> 
>        error_setg(errp, "PCI: slot %d is not valid for %s:"
>                   " PCI Express devices can only be plugged into slot 0")
> 
> and then you don't really need a comment.
> 
> 
>> +                   PCI_SLOT(devfn), name);
>> +        return NULL;
>>     }
>> 
>>     pci_dev->devfn = devfn;
>> -- 
>> 2.39.1