Re: [PATCH 4/9] vpci/header: Add and remove register handlers dynamically

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.09.2021 14:16, Oleksandr Andrushchenko wrote:
> 
> On 07.09.21 14:49, Jan Beulich wrote:
>> On 07.09.2021 13:10, Oleksandr Andrushchenko wrote:
>>> On 07.09.21 13:43, Jan Beulich wrote:
>>>> On 07.09.2021 12:11, Oleksandr Andrushchenko wrote:
>>>>> On 06.09.21 17:11, Jan Beulich wrote:
>>>>>> On 03.09.2021 12:08, Oleksandr Andrushchenko wrote:
>>>>>>> @@ -593,6 +625,36 @@ static int init_bars(struct pci_dev *pdev)
>>>>>>>     }
>>>>>>>     REGISTER_VPCI_INIT(init_bars, VPCI_PRIORITY_MIDDLE);
>>>>>>>     
>>>>>>> +int vpci_bar_add_handlers(const struct domain *d, struct pci_dev *pdev)
>>>>>>> +{
>>>>>>> +    int rc;
>>>>>>> +
>>>>>>> +    /* Remove previously added registers. */
>>>>>>> +    vpci_remove_device_registers(pdev);
>>>>>>> +
>>>>>>> +    /* It only makes sense to add registers for hwdom or guest domain. 
>>>>>>> */
>>>>>>> +    if ( d->domain_id >= DOMID_FIRST_RESERVED )
>>>>>>> +        return 0;
>>>>>>> +
>>>>>>> +    if ( is_hardware_domain(d) )
>>>>>>> +        rc = add_bar_handlers(pdev, true);
>>>>>>> +    else
>>>>>>> +        rc = add_bar_handlers(pdev, false);
>>>>>>        rc = add_bar_handlers(pdev, is_hardware_domain(d));
>>>>> Indeed, thank you ;)
>>>>>>> +    if ( rc )
>>>>>>> +        gprintk(XENLOG_ERR,
>>>>>>> +            "%pp: failed to add BAR handlers for dom%d\n", &pdev->sbdf,
>>>>>>> +            d->domain_id);
>>>>>> Please use %pd and correct indentation. Logging the error code might
>>>>>> also help some in diagnosing issues.
>>>>> Sure, I'll change it to %pd
>>>>>>     Further I'm not sure this is a
>>>>>> message we want in release builds
>>>>> Why not?
>>>> Excess verbosity: If we have such here, why not elsewhere on error paths?
>>>> And I hope you agree things will get too verbose if we had such (about)
>>>> everywhere?
>>> Agree, will change it to gdprintk
>>>>>>     - perhaps gdprintk()?
>>>>> I'll change if we decide so
>>>>>>> +    return rc;
>>>>>>> +}
>>>>>>> +
>>>>>>> +int vpci_bar_remove_handlers(const struct domain *d, struct pci_dev 
>>>>>>> *pdev)
>>>>>>> +{
>>>>>>> +    /* Remove previously added registers. */
>>>>>>> +    vpci_remove_device_registers(pdev);
>>>>>>> +    return 0;
>>>>>>> +}
>>>>>> Also - in how far is the goal of your work to also make vPCI work for
>>>>>> x86 DomU-s? If that's not a goal
>>>>> It is not, unfortunately. The goal is not to break x86 and to enable Arm
>>>>>> , I'd like to ask that you limit the
>>>>>> introduction of code that ends up dead there.
>>>>> What's wrong with this function even if it is a one-liner?
>>>> The comment is primarily on the earlier function, and then extends to
>>>> this one.
>>>>
>>>>> This way we have a pair vpci_bar_add_handlers/vpci_bar_remove_handlers
>>>>> and if I understood correctly you suggest 
>>>>> vpci_bar_add_handlers/vpci_remove_device_registers?
>>>>> What would we gain from that, but yet another secret knowledge that in 
>>>>> order
>>>>> to remove BAR handlers one needs to call vpci_remove_device_registers
>>>>> while I would personally expect to call vpci_bar_add_handlers' 
>>>>> counterpart,
>>>>> vpci_remove_device_registers namely.
>>>> This is all fine. Yet vpci_bar_{add,remove}_handlers() will, aiui, be
>>>> dead code on x86.
>>> vpci_bar_add_handlers will be used by x86 PVH Dom0
>> Where / when? You add a call from vpci_assign_device(), but besides that
>> also being dead code on x86 (for now), you can't mean that because
>> vpci_deassign_device() also calls vpci_bar_remove_handlers().
> 
> You are right here and both add/remove are not used on x86 PVH Dom0.
> 
> I am sorry for wasting your time
> 
>>
>>>>    Hence there should be an arrangement allowing the
>>>> compiler to eliminate this dead code.
>>> So, the only dead code for x86 here will be vpci_bar_remove_handlers. Yet.
>>> Because I hope x86 to gain guest support for PVH Dom0 sooner or later.
>>>
>>>>    Whether that's enclosing these
>>>> by "#ifdef" or adding early "if(!IS_ENABLED(CONFIG_*))" is secondary.
>>>> This has a knock-on effect on other functions as you certainly realize:
>>>> The compiler seeing e.g. the 2nd argument to the add-BARs function
>>>> always being true allows it to instantiate just a clone of the original
>>>> function with the respective conditionals removed.
>>> With the above (e.g. add is going to be used, but not remove) do you
>>> think it is worth playing with ifdef's to strip that single function and add
>>> a piece of spaghetti code to save a bit?
>> No, that I agree wouldn't be worth it.
>>
>>> What would that ifdef look like,
>>> e.g. #ifdef CONFIG_ARM or #ifndef CONFIG_X86 && any other platform, but ARM?
>> A new setting, preferably; e.g. VCPU_UNPRIVILEGED, to be "select"ed by
>> architectures as functionality gets enabled.
> 
> So, as add/remove are only needed for Arm at the moment
> you suggest I add VCPU_UNPRIVILEGED to Arm's Kconfig to enable
> compiling vpci_bar_add_handlers/vpci_bar_remove_handlers?
> To me this is more about vPCI's support for guests, so should we probably 
> call it
> VPCI_XXX instead? E.g. VPCI_HAS_GUEST_SUPPORT or something which
> will reflect the nature of the code being gated? VCPU_UNPRIVILEGED sounds
> like not connected to vpci to me

And validly so - my fingers didn't type what the brain told them. I've
meant VPCI_UNPRIVILEGED. I would also be okay with HAS_VPCI_GUEST_SUPPORT
(i.e. not exactly what you've suggested), for example.

Jan