Re: [PATCH v6 09/13] vpci/header: emulate PCI_COMMAND register for guests

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>]

On 08.02.22 11:33, Jan Beulich wrote:
> On 08.02.2022 09:13, Oleksandr Andrushchenko wrote:
>> On 04.02.22 16:25, Jan Beulich wrote:
>>> On 04.02.2022 07:34, Oleksandr Andrushchenko wrote:
>>>> --- a/xen/drivers/vpci/header.c
>>>> +++ b/xen/drivers/vpci/header.c
>>>> @@ -454,6 +454,22 @@ static void cmd_write(const struct pci_dev *pdev, 
>>>> unsigned int reg,
>>>>            pci_conf_write16(pdev->sbdf, reg, cmd);
>>>>    }
>>>>    
>>>> +static void guest_cmd_write(const struct pci_dev *pdev, unsigned int reg,
>>>> +                            uint32_t cmd, void *data)
>>>> +{
>>>> +    /* TODO: Add proper emulation for all bits of the command register. */
>>>> +
>>>> +#ifdef CONFIG_HAS_PCI_MSI
>>>> +    if ( pdev->vpci->msi->enabled || pdev->vpci->msix->enabled )
>>>> +    {
>>>> +        /* Guest wants to enable INTx. It can't be enabled if MSI/MSI-X 
>>>> enabled. */
>>>> +        cmd |= PCI_COMMAND_INTX_DISABLE;
>>>> +    }
>>>> +#endif
>>>> +
>>>> +    cmd_write(pdev, reg, cmd, data);
>>>> +}
>>> It's not really clear to me whether the TODO warrants this being a
>>> separate function. Personally I'd find it preferable if the logic
>>> was folded into cmd_write().
>> Not sure cmd_write needs to have guest's logic. And what's the
>> profit? Later on, when we decide how PCI_COMMAND can be emulated
>> this code will live in guest_cmd_write anyways
> Why "will"? There's nothing conceptually wrong with putting all the
> emulation logic into cmd_write(), inside an if(!hwdom) conditional.
> If and when we gain CET-IBT support on the x86 side (and I'm told
> there's an Arm equivalent of this), then to make this as useful as
> possible it is going to be desirable to limit the number of functions
> called through function pointers. You may have seen Andrew's huge
> "x86: Support for CET Indirect Branch Tracking" series. We want to
> keep down the number of such annotations; the vast part of the series
> is about adding of such.
Well, while I see nothing bad with that, from the code organization
it would look a bit strange: we don't differentiate hwdom in vpci
handlers, but instead provide one for hwdom and one for guests.
While I understand your concern I still think that at the moment
it will be more in line with the existing code if we provide a dedicated
handler.

Once we are all set with the handlers we may want performing a refactoring
with limiting the number of register handlers.

@Roger, what's your view on this?
> Jan
>
>
Thank you,
Oleksandr