Re: [PATCH v5 09/14] vpci/header: emulate PCI_COMMAND register for guests

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.02.2022 14:47, Oleksandr Andrushchenko wrote:
>> On 02.02.2022 13:49, Oleksandr Andrushchenko wrote:
>>> On 13.01.22 12:50, Roger Pau Monné wrote:
>>>> On Thu, Nov 25, 2021 at 01:02:46PM +0200, Oleksandr Andrushchenko wrote:
>>>>> --- a/xen/drivers/vpci/header.c
>>>>> +++ b/xen/drivers/vpci/header.c
>>>>> @@ -491,6 +491,22 @@ 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 )
>>>> You need to check for MSI-X also, pdev->vpci->msix->enabled.
>>> Indeed, thank you
>>>>> +    {
>>>>> +        /* Guest wants to enable INTx. It can't be enabled if MSI/MSI-X 
>>>>> enabled. */
>>>>> +        cmd |= PCI_COMMAND_INTX_DISABLE;
>>>> You will also need to make sure PCI_COMMAND_INTX_DISABLE is set in the
>>>> command register when attempting to enable MSI or MSIX capabilities.
>>> Isn't it enough that we just check above if MSI/MSI-X enabled then make
>>> sure INTX disabled? I am not following you here on what else needs to
>>> be done.
>> No, you need to deal with the potentially bad combination on both
>> paths - command register writes (here) and MSI/MSI-X control register
>> writes (which is what Roger points you at). I would like to suggest
>> to consider simply forcing INTX_DISABLE on behind the guest's back
>> for those other two paths.
> Do you suggest that we need to have some code which will
> write PCI_COMMAND while we write MSI/MSI-X control register
> for that kind of consistency? E.g. control register handler will
> need to write to PCI_COMMAND and go through emulation for
> guests?

Either check or write, yes. Since you're setting the bit here behind
the guest's back, setting it on the other paths as well would only
look consistent to me.

> If so, why didn't we have that before?

Because we assume Dom0 to be behaving itself.

Jan