Re: [PATCH v5 5/5] vPCI: re-init extended-capabilities when MMCFG availability changed

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.03.2026 16:06, Roger Pau Monné wrote:
> On Wed, Feb 25, 2026 at 12:44:44PM +0100, Jan Beulich wrote:
>> When Dom0 informs us about MMCFG usability, this may change whether
>> extended capabilities are available (accessible) for devices. Zap what
>> might be on record, and re-initialize things.
>>
>> No synchronization is added for the case where devices may already be in
>> use. That'll need sorting when (a) DomU support was added and (b) DomU-s
>> may run already while Dom0 / hwdom still boots (dom0less, Hyperlaunch).
>>
>> vpci_cleanup_capabilities() also shouldn't have used
>> pci_find_ext_capability(), as already when the function was introduced
>> extended config space may not have been (properly) accessible anymore,
>> no matter whether it was during init. Extended capability cleanup hooks
>> need to cope with being called when the respective capability doesn't
>> exist (and hence the corresponding ->init() hook was never called).
>>
>> Fixes: 70e6dace747e ("vpci: Use cleanup to free capability resource during 
>> deassign")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> vpci_reinit_ext_capabilities()'es return value is checked only to log an
>> error; it doesn't feel quite right to fail the hypercall because of this.
>> Roger brought up the idea of de-assigning the device in such a case, but
>> if a driver doesn't use extended capabilities the device would likely
>> continue to work fine, for Dom0 this probably wouldn't be quite right
>> anyway, and it's also unclear whether calling deassign_device() could be
>> done from this context. Something like what pci_check_disable_device()
>> does may be an option, if we really think we need to "break" the device.
> 
> We may want to add a note there, stating that we have considered all
> possible options, and hiding the capability and hoping the owner
> domain would continue to work as expected seems the less bad of all of
> them?

I'll see what I can do.

>> The use of is_hardware_domain() in vpci_cleanup_capabilities() was
>> uncommented and hence is left so. Shouldn't there be a DomU-related TODO
>> or FIXME?
> 
> Hm, yes, possibly.  I think limiting extended space availability to
> the hardware domain only has been done "just" because we have no
> extended capabilities to expose to domUs so far, and I don't think we
> even setup the extended capability list in the domU case.

Considering how many things there are to be done for DomU support, I
think it would help if every place where e.g. is_hardware_domain() is
used only as surrogate would be properly annotated. Or perhaps properly
named predicates could be introduced right away, so one can actually go
hunt for all of them. Then again is_hardware_domain() is also something
one can go hunt for.

>> @@ -349,22 +352,23 @@ int vpci_init_capabilities(struct pci_de
>>      return 0;
>>  }
>>  
>> -void vpci_cleanup_capabilities(struct pci_dev *pdev)
>> +void vpci_cleanup_capabilities(struct pci_dev *pdev, bool ext_only)
>>  {
> 
> You could short-circuit the function here, ie:
> 
> if ( ext_only && !is_hardware_domain(pdev->domain) )
>     return;
> 
> But I'm not sure that would simplify the code of the function much?
> Likewise for vpci_init_capabilities().

Such a short-circuit would need replacing / dropping once DomU support is
added. I was hoping the chosen arrangement would make for a little less
churn at that time. I'll listen to your advice, though, just that the
question gives the impression you're not quite sure either.

>>      for ( unsigned int i = 0; i < NUM_VPCI_INIT; i++ )
>>      {
>>          const vpci_capability_t *capability = &__start_vpci_array[i];
>>          const unsigned int cap = capability->id;
>> -        unsigned int pos = 0;
>>  
>>          if ( !capability->cleanup )
>>              continue;
>>  
>> -        if ( !capability->is_ext )
>> -            pos = pci_find_cap_offset(pdev->sbdf, cap);
>> -        else if ( is_hardware_domain(pdev->domain) )
>> -            pos = pci_find_ext_capability(pdev, cap);
>> -        if ( pos )
>> +        /*
>> +         * Cannot call pci_find_ext_capability() here, as extended config
>> +         * space may (no longer) be accessible.
>> +         */
>> +        if ( capability->is_ext
>> +             ? is_hardware_domain(pdev->domain)
>> +             : !ext_only && pci_find_cap_offset(pdev->sbdf, cap) )
> 
> Given the changes you have done to the reBAR cleanup function, we
> could even call capability->cleanup() on domUs, as the handler has to
> deal with uninitialized capabilities either way?

Hmm, yes, looks like we could.

>> @@ -376,6 +380,28 @@ void vpci_cleanup_capabilities(struct pc
>>      }
>>  }
>>  
>> +int vpci_reinit_ext_capabilities(struct pci_dev *pdev)
>> +{
>> +    if ( !pdev->vpci )
>> +        return 0;
>> +
>> +    /*
>> +     * FIXME: DomU support is missing.  For already running domains we may
>> +     * need to pause them around the entire re-evaluation of extended config
>> +     * space accessibility.
>> +     */
>> +    if ( pdev->domain )
>> +        ASSERT(pdev->domain == hardware_domain || pdev->domain == dom_io);
> 
> Is this to cope around races?  I don't think it's a valid state to
> have pdev->vpci != NULL and pdev->domain == NULL?

Knowing that I had seen pdev-s with NULL domains, I'm perhaps overly cautious.
Yes, ->vpci being non-NULL ought to demand a proper owner.

> Neither you can have pdev->domain == dom_io and pdev->vpci != NULL?

Same here, looks like I went too far.

>> +
>> +    vpci_cleanup_capabilities(pdev, true);
>> +
>> +    if ( vpci_remove_registers(pdev->vpci, PCI_CFG_SPACE_SIZE,
>> +                               PCI_CFG_SPACE_EXP_SIZE - PCI_CFG_SPACE_SIZE) 
>> )
>> +        ASSERT_UNREACHABLE();
> 
> Ideally this would better be done the other way around.  We first
> remove the handlers, and the cleanup the capabilities.  Just to ensure
> no stray handler could end up having cached references to data that's
> been freed by vpci_cleanup_capabilities().

And maybe not just that: For the hwdom case cleanup_rebar() adds new handlers,
which we'd wrongly purge again right away. (Because we pass "false" for "hide",
this isn't an active issue right now.)

> And we should take the write_lock(&pdev->domain->pci_lock).

Now this is a request that I'm struggling with some. I can see that callers
of vpci_{init,cleanup}_capabilities() assert that the lock is being held, yet
it's not quite clear to me why that's needed. Shouldn't vPCI internals all
synchronize on the vPCI lock of the domain?

Jan