Re: [PATCH v4 03/10] vpci/header: Emulate extended capability list for dom0

["Chen, Jiqian" <Jiqian.Chen@xxxxxxx>]

On 2025/5/18 22:20, Jan Beulich wrote:
> On 09.05.2025 11:05, Jiqian Chen wrote:
>> @@ -827,6 +827,34 @@ static int vpci_init_capability_list(struct pci_dev 
>> *pdev)
>>                                                   PCI_STATUS_RSVDZ_MASK);
>>  }
>>  
>> +static int vpci_init_ext_capability_list(struct pci_dev *pdev)
>> +{
>> +    unsigned int pos = PCI_CFG_SPACE_SIZE, ttl = 480;
> 
> The ttl value exists (in the function you took it from) to make sure
> the loop below eventually ends. That is, to be able to kind of
> gracefully deal with loops in the linked list. Such loops, however,
> would ...
> 
>> +    if ( !is_hardware_domain(pdev->domain) )
>> +        /* Extended capabilities read as zero, write ignore for guest */
>> +        return vpci_add_register(pdev->vpci, vpci_read_val, NULL,
>> +                                 pos, 4, (void *)0);
>> +
>> +    while ( pos >= PCI_CFG_SPACE_SIZE && ttl-- )
>> +    {
>> +        uint32_t header = pci_conf_read32(pdev->sbdf, pos);
>> +        int rc;
>> +
>> +        if ( !header )
>> +            return 0;
>> +
>> +        rc = vpci_add_register(pdev->vpci, vpci_read_val, vpci_hw_write32,
>> +                               pos, 4, (void *)(uintptr_t)header);
> 
> ... mean we may invoke this twice for the same capability. Such
> a secondary invocation would fail with -EEXIST, causing device init
> to fail altogether. Which is kind of against our aim of exposing
> (in a controlled manner) as much of the PCI hardware as possible.
May I know what situation that can make this twice for one capability when 
initialization?
Does hardware capability list have a cycle?

> 
> Imo we ought to be using a bitmap to detect the situation earlier
> and hence to be able to avoid redundant register addition. Thoughts?
Can we just let it go forward and continue to add register for next capability 
when rc == -EXIST, instead of returning error ?

> 
> Jan

-- 
Best regards,
Jiqian Chen.