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

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

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

>
> Jan
>
Thank you,

Oleksandr