Re: [Qemu-devel] [RFC PATCH] qemu pci: pci_add_capability enhancement to prevent damaging config space

[Alexey Kardashevskiy <aik@ozlabs.ru>]

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@ozlabs.ru>
>> ---
>> 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.
> 
> 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.

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?


>> +        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;
>> +    }
>>     memset(pdev->used + offset, 0xFF, size);
>>     /* Make capability read-only by default */
>>     memset(pdev->wmask + offset, 0, size);


-- 
With best regards

Alexey Kardashevskiy