On 11.05.2012, at 14:47, Alexey Kardashevskiy wrote:
> 11.05.2012 20:52, Alexander Graf написал:
>>
>> On 11.05.2012, at 08:45, Alexey Kardashevskiy wrote:
>>
>>> Normally the pci_add_capability is called on devices to add new
>>> capability. This is ok for emulated devices which capabilities list
>>> is being built by QEMU.
>>>
>>> In the case of VFIO the capability may already exist and adding new
>>> capability into the beginning of the linked list may create a loop.
>>>
>>> For example, the old code destroys the following config
>>> of PCIe Intel E1000E:
>>>
>>> before adding PCI_CAP_ID_MSI (0x05):
>>> 0x34: 0xC8
>>> 0xC8: 0x01 0xD0
>>> 0xD0: 0x05 0xE0
>>> 0xE0: 0x10 0x00
>>>
>>> after:
>>> 0x34: 0xD0
>>> 0xC8: 0x01 0xD0
>>> 0xD0: 0x05 0xC8
>>> 0xE0: 0x10 0x00
>>>
>>> As result capabilities 0x01 and 0x05 point to each other.
>>>
>>> The proposed patch does not change capability pointers when
>>> the same type capability is about to add.
>>>
>>> Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
>>> ---
>>> hw/pci.c | 10 ++++++----
>>> 1 files changed, 6 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/hw/pci.c b/hw/pci.c
>>> index aa0c0b8..1f7c924 100644
>>> --- a/hw/pci.c
>>> +++ b/hw/pci.c
>>> @@ -1794,10 +1794,12 @@ int pci_add_capability(PCIDevice *pdev, uint8_t cap_id,
>>> }
>>>
>>> config = pdev->config + offset;
>>> - config[PCI_CAP_LIST_ID] = cap_id;
>>> - config[PCI_CAP_LIST_NEXT] = pdev->config[PCI_CAPABILITY_LIST];
>>> - pdev->config[PCI_CAPABILITY_LIST] = offset;
>>> - pdev->config[PCI_STATUS] |= PCI_STATUS_CAP_LIST;
>>> + if (config[PCI_CAP_LIST_ID] != cap_id) {
>>
>> This doesn't scale. Capabilities are a list of CAPs. You'll have to do a loop through all capabilities, check if the one you want to add is there already and if so either
>> * replace the existing one or
>> * drop out and not write the new one in.
* hw_error :)
>>
>> I'm not sure which way would be more natural.
>
> There is a third option - add another function, lets call it
> pci_fixup_capability() which would do whatever pci_add_capability() does
> but won't touch list pointers.
What good is a function that breaks internal consistency?
> When vfio, pci_add_capability() is called from the code which knows
> exactly that the capability exists and where it is and it calls
> pci_add_capability() based on this knowledge so doing additional loops
> just for imaginery scalability is a bit weird, no?
Not sure I understand your proposal. The more generic a framework is, the better, no? In this code path we don't care about speed. We only care about consistency and reliability.
Alex
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