Re: [PATCH v6 4/8] vpci: Hide extended capability when it fails to initialize

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.06.2025 11:29, Jiqian Chen wrote:
> +static int vpci_ext_capability_hide(struct pci_dev *pdev, unsigned int cap)
> +{
> +    const unsigned int offset = pci_find_ext_capability(pdev->sbdf, cap);
> +    struct vpci_register *r, *prev_r;
> +    struct vpci *vpci = pdev->vpci;
> +    uint32_t header, pre_header;
> +
> +    if ( offset < PCI_CFG_SPACE_SIZE )
> +    {
> +        ASSERT_UNREACHABLE();
> +        return 0;
> +    }
> +
> +    spin_lock(&vpci->lock);
> +    r = vpci_get_register(vpci, offset, 4);
> +    if ( !r )
> +    {
> +        spin_unlock(&vpci->lock);
> +        return -ENODEV;
> +    }
> +
> +    header = (uint32_t)(uintptr_t)r->private;
> +    if ( offset == PCI_CFG_SPACE_SIZE )
> +    {
> +        if ( PCI_EXT_CAP_NEXT(header) <= PCI_CFG_SPACE_SIZE )
> +            r->private = (void *)(uintptr_t)0;
> +        else
> +            /*
> +             * The first extended capability (0x100) can not be removed from
> +             * the linked list, so instead mask its capability ID to return 0
> +             * and force OSes to skip it.
> +             */
> +            r->private = (void *)(uintptr_t)(header & 
> ~PCI_EXT_CAP_ID(header));

Can we rely on OSes recognizing ID 0 as "just skip"? Since the size isn't 
encoded
in the header, there might be issues lurking here.

Jan