Re: [Xen-devel] [PATCH for Xen 4.13] x86/msi: Don't panic if msix capability is missing

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.09.2019 23:24, Chao Gao wrote:
> --- a/xen/arch/x86/msi.c
> +++ b/xen/arch/x86/msi.c
> @@ -1265,7 +1265,13 @@ int pci_msi_conf_write_intercept(struct pci_dev *pdev, 
> unsigned int reg,
>          pos = entry ? entry->msi_attrib.pos
>                      : pci_find_cap_offset(seg, bus, slot, func,
>                                            PCI_CAP_ID_MSIX);
> -        ASSERT(pos);
> +        if ( unlikely(!pos) )
> +        {
> +            printk_once(XENLOG_WARNING
> +                        "%04x:%02x:%02x.%u MSI-X capability is missing\n",
> +                        seg, bus, slot, func);
> +            return -EAGAIN;
> +        }

Besides agreeing with Roger's comments, whose access do we
intercept here at the time you observe the operation above
producing a zero "pos"? If it's Dom0, then surely there's a bug
in Dom0 doing the access in the first place when a reset hasn't
completed yet? If it's a DomU, then is the reset happening
behind _its_ back as well (which is not going to end well)?

Jan

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