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

[Michael S. Tsirkin]

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.

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