Re: [PATCH v6 7/8] vpci/msi: Free MSI resources when init_msi() fails

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.06.2025 09:16, Chen, Jiqian wrote:
> On 2025/6/24 18:17, Jan Beulich wrote:
>> On 24.06.2025 11:49, Chen, Jiqian wrote:
>>> On 2025/6/18 22:45, Jan Beulich wrote:
>>>> On 12.06.2025 11:29, Jiqian Chen wrote:
>>>>> --- a/xen/drivers/vpci/msi.c
>>>>> +++ b/xen/drivers/vpci/msi.c
>>>>> @@ -193,6 +193,33 @@ static void cf_check mask_write(
>>>>>      msi->mask = val;
>>>>>  }
>>>>>  
>>>>> +static int cf_check cleanup_msi(struct pci_dev *pdev)
>>>>> +{
>>>>> +    int rc;
>>>>> +    unsigned int end, size;
>>>>> +    struct vpci *vpci = pdev->vpci;
>>>>> +    const unsigned int msi_pos = pdev->msi_pos;
>>>>> +    const unsigned int ctrl = msi_control_reg(msi_pos);
>>>>> +
>>>>> +    if ( !msi_pos || !vpci->msi )
>>>>> +        return 0;
>>>>> +
>>>>> +    if ( vpci->msi->masking )
>>>>> +        end = msi_pending_bits_reg(msi_pos, vpci->msi->address64);
>>>>> +    else
>>>>> +        end = msi_mask_bits_reg(msi_pos, vpci->msi->address64) - 2;
>>>>> +
>>>>> +    size = end - ctrl;
>>>>> +
>>>>> +    rc = vpci_remove_registers(vpci, ctrl, size);
>>>>> +    if ( rc )
>>>>> +        return rc;
>>>>
>>>> This is a difficult one: It's not a good idea to simply return here, yet
>>>> at the same time the handling of the register we're unable to remove may
>>>> still require e.g. ...
>>>>
>>>>> +    XFREE(vpci->msi);
>>>>
>>>> ... this. There may therefore be more work required, such that in the
>>>> end we're able to ...
>>>>
>>>>> +    return vpci_add_register(pdev->vpci, vpci_hw_read16, NULL, ctrl, 2, 
>>>>> NULL);
>>>>
>>>> ... try this at least on a best effort basis.
>>>>
>>>> More generally: I don't think failure here (or in other .cleanup hook
>>>> functions) may go entirely silently.
>>> Does below meet your modification expectations?
>>
>> Not sure, sorry. By "more" I really meant "more" (which may just be code
>> auditing, results of which would need writing down, but which may also
>> involve further code changes; see below).
>>
>>>     rc = vpci_remove_registers(vpci, ctrl, size);
>>>     if ( rc )
>>>         printk(XENLOG_ERR "%pd %pp: remove msi handlers fail rc=%d\n",
>>>                pdev->domain, &pdev->sbdf, rc);
>>>
>>>     XFREE(vpci->msi);
>>
>> As I tried to indicate in my earlier reply, the freeing of this struct is
>> safe only if the failure above would not leave any register handlers in
>> place which still (without appropriate checking) use this struct.
> Hmm, but all handlers added in init_msi() use this struct.
> So it doesn't exist the case that when above unable to remove all handlers 
> and still require xfree this struct.

Well, in the end you say in different words what I did say, if I understand
correctly. There are several options how to deal with that. One might be to
have those handlers recognize the lack of that pointer, and behave like ...

>>>     /*
>>>      * The driver may not traverse the capability list and think device
>>>      * supports MSI by default. So here let the control register of MSI
>>>      * be Read-Only is to ensure MSI disabled.
>>>      */
>>>     rc = vpci_add_register(vpci, vpci_hw_read16, NULL, ctrl, 2, NULL);

... what is tried to be put in place here (and like "no handler installed"
for other registers).

>> You're losing the earlier error here, if there was one. If this one
>> succeeds, ...
> But if return the earlier error to the caller, this device will be unusable, 
> then still adding this handler when above failing to remove handlers is 
> useless.

True, yet that's the case also with your code if removing the ctrl handler
failed, as then the attempt above to add another handler would also fail.

I don't know what the best approach is (I did suggest one above, albeit
that's not quite complete yet as to the behavior here); I merely observed
that the behavior as you have it doesn't look quite right / consistent.

Jan

>>>     if ( rc )
>>>         printk(XENLOG_ERR "%pd %pp: add dummy msi ctrl handler fail 
>>> rc=%d\n",
>>>                pdev->domain, &pdev->sbdf, rc);
>>>
>>>     return rc;
>>
>> ... the caller would (wrongly) get success back.
>>
>> Jan
>