Re: virtio capabilities

[Michael S. Tsirkin]

On Fri, Dec 13, 2019 at 05:05:05PM +1100, Alexey Kardashevskiy wrote:
> Hi!
> 
> I am having an issue with capabilities (hopefully the chunk formatting
> won't break).
> 
> The problem is that when virtio_pci_find_capability() reads
> pci_find_capability(dev, PCI_CAP_ID_VNDR), 0 is returned; if repeated,
> it returns a valid number (0x84). Timing seems to matter. pci_cfg_read
> trace shows that that first time read does not reach QEMU but others do
> reach QEMU and return what is expected.
> 
> How to debug this, any quick ideas?
> The config space is not a MMIO BAR
> or KVM memory slot or anything like this, right? :) Thanks,

Depends on the platform.

E.g. on x86, when using cf8/cfc pair, if guest doesn't
have a lock around programming the pair of registers,
then one access can conflict with another one.

When using express it's MMIO so shouldn't be a problem.

> 
> [    3.489492] ___K___ (0) virtio_pci_modern_probe 642
> [    3.489697] ___K___ (0) virtio_pci_find_capability 492: FIND a cap
> [    3.490070] ___K___ (0) virtio_pci_find_capability 494: cap is at 0
> [    3.490335] ___K___ (0) virtio_pci_find_capability 492: FIND a cap
> 10909@1576216763.643271:pci_cfg_read virtio-net-pci 00:0 @0x6 -> 0x10
> 10909@1576216763.643431:pci_cfg_read virtio-net-pci 00:0 @0x34 -> 0x98
> 10909@1576216763.643591:pci_cfg_read virtio-net-pci 00:0 @0x98 -> 0x8411
> 10909@1576216763.643747:pci_cfg_read virtio-net-pci 00:0 @0x84 -> 0x7009
> [    3.491264] ___K___ (0) virtio_pci_find_capability 494: cap is at 132
> 10909@1576216763.644140:pci_cfg_read virtio-net-pci 00:0 @0x87 -> 0x5
> 10909@1576216763.644287:pci_cfg_read virtio-net-pci 00:0 @0x88 -> 0x0
> [    3.491803] ___K___ (0) virtio_pci_find_capability 506: 5 0
> 10909@1576216763.644632:pci_cfg_read virtio-net-pci 00:0 @0x85 -> 0x70
> 10909@1576216763.644786:pci_cfg_read virtio-net-pci 00:0 @0x70 -> 0x6009
> 10909@1576216763.644942:pci_cfg_read virtio-net-pci 00:0 @0x73 -> 0x2
> 10909@1576216763.645092:pci_cfg_read virtio-net-pci 00:0 @0x74 -> 0x4
> [    3.492607] ___K___ (0) virtio_pci_find_capability 506: 2 4
> 
> 
> 
> 
> 
> diff --git a/drivers/virtio/virtio_pci_modern.c
> b/drivers/virtio/virtio_pci_modern.c
> index 7abcc50838b8..85b2a7ce96e9 100644
> --- a/drivers/virtio/virtio_pci_modern.c
> +++ b/drivers/virtio/virtio_pci_modern.c
> @@ -486,9 +486,14 @@ static const struct virtio_config_ops
> virtio_pci_config_ops = {
>  static inline int virtio_pci_find_capability(struct pci_dev *dev, u8
> cfg_type,
>                                              u32 ioresource_types, int
> *bars)
>  {
> -       int pos;
> +       int pos = 0;// = pci_find_capability(dev, PCI_CAP_ID_VNDR);
> 
> -       for (pos = pci_find_capability(dev, PCI_CAP_ID_VNDR);
> +       while (!pos) {
> +               pr_err("___K___ (%u) %s %u: FIND a cap\n",
> smp_processor_id(), __func__, __LINE__);
> +               pos = pci_find_capability(dev, PCI_CAP_ID_VNDR);
> +               pr_err("___K___ (%u) %s %u: cap is at %d\n",
> smp_processor_id(), __func__, __LINE__, pos);
> +       }
> +       for (;
>              pos > 0;
>              pos = pci_find_next_capability(dev, pos, PCI_CAP_ID_VNDR)) {
>                 u8 type, bar;
> 
> 
> -- 
> Alexey