Re: [PATCH v3 2/6] PCI: determine whether a device has extended config space

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.01.2026 15:09, Roger Pau Monné wrote:
> On Thu, Jan 29, 2026 at 02:08:27PM +0100, Jan Beulich wrote:
>> Legacy PCI devices don't have any extended config space. Reading any part
>> thereof may return all ones or other arbitrary data, e.g. in some cases
>> base config space contents repeatedly.
>>
>> Logic follows Linux 6.19-rc's pci_cfg_space_size(), albeit leveraging our
>> determination of device type; in particular some comments are taken
>> verbatim from there. Like with Linux'es CONFIG_PCI_QUIRKS, only the alias
>> detection logic is covered by the new "pci=no-quirks". The singular access
>> at PCI_CFG_SPACE_SIZE is left unconditional.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> ---
>> The warning near the bottom of pci_check_extcfg() may be issued multiple
>> times for a single device now. Should we try to avoid that?
>>
>> Note that no vPCI adjustments are done here, but they're going to be
>> needed: Whatever requires extended capabilities will need re-
>> evaluating / newly establishing / tearing down in case an invocation of
>> PHYSDEVOP_pci_mmcfg_reserved alters global state.
>> ---
>> v3: Add command line (sub-)option.
>> v2: Major re-work to also check upon PHYSDEVOP_pci_mmcfg_reserved
>>     invocation.
>>
>> --- a/docs/misc/xen-command-line.pandoc
>> +++ b/docs/misc/xen-command-line.pandoc
>> @@ -2009,12 +2009,21 @@ Only effective if CONFIG_PARTIAL_EMULATI
>>  behavior.**
>>  
>>  ### pci
>> -    = List of [ serr=<bool>, perr=<bool> ]
>> +    = List of [ serr=<bool>, perr=<bool>, quirks=<bool> ]
>> +
>> +* `serr` and `perr`
>>  
>>      Default: Signaling left as set by firmware.
>>  
>> -Override the firmware settings, and explicitly enable or disable the
>> -signalling of PCI System and Parity errors.
>> +  Override the firmware settings, and explicitly enable or disable the
>> +  signalling of PCI System and Parity errors.
>> +
>> +* `quirks`
>> +
>> +    Default: `on`
>> +
>> +  In its negative form, allows to suppress certain quirk workarounds, in 
>> case
>> +  they cause issues.
> 
> Not that I oppose to this, but I've assumed that you would introduce
> an option to fallback to the previous behavior where Xen would just
> assume extended space to be accessible.

But that would be wrong. It didn't even occur to me that you could have
wanted that.

Jan