Re: [PATCH v2 3/8] vpci/header: Emulate extended capability list for host

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

On 2025/4/15 17:49, Jan Beulich wrote:
> On 09.04.2025 08:45, Jiqian Chen wrote:
>> --- a/xen/drivers/vpci/header.c
>> +++ b/xen/drivers/vpci/header.c
>> @@ -815,6 +815,39 @@ static int vpci_init_capability_list(struct pci_dev 
>> *pdev)
>>      return rc;
>>  }
>>  
>> +static int vpci_init_ext_capability_list(struct pci_dev *pdev)
>> +{
>> +    int rc;
>> +    u32 header;
>> +    unsigned int pos = 0x100U, ttl = 480;
>> +
>> +    if ( !is_hardware_domain(pdev->domain) )
>> +    {
>> +        /* Extended capabilities read as zero, write ignore */
>> +        rc = vpci_add_register(pdev->vpci, vpci_read_val, NULL,
>> +                               pos, 4, (void *)0);
>> +        if ( rc )
>> +            return rc;
>> +    }
>> +
>> +    while ( pos && ttl-- )
>> +    {
>> +        header = pci_conf_read32(pdev->sbdf, pos);
>> +
>> +        rc = vpci_add_register(pdev->vpci, vpci_read_val, NULL,
>> +                               pos, 4, (void *)(uintptr_t)header);
>> +        if ( rc )
>> +            return rc;
>> +
>> +        if ( (header == 0) || (header == -1) )
>> +            return 0;
> 
> I realize pci_find_next_ext_capability() also has such a check, but even
> there it's not really clear to me why compare not only against 0, but also
> again -1 (aka ~0).
Thank you for raising this question.
When I coded this part, I also had this confuse since 
pci_find_next_ext_capability() has this check,
so I chose to keep the same check.
Do you think I need to remove this -1 check?

> 
> Jan

-- 
Best regards,
Jiqian Chen.