Re: [PATCH v3 08/11] vpci/header: Emulate PCI_COMMAND register for guests

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.10.2021 12:52, Roger Pau Monné wrote:
> On Thu, Sep 30, 2021 at 10:52:20AM +0300, Oleksandr Andrushchenko wrote:
>> --- a/xen/drivers/vpci/header.c
>> +++ b/xen/drivers/vpci/header.c
>> @@ -451,6 +451,32 @@ 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. */
>> +
>> +    if ( (cmd & PCI_COMMAND_INTX_DISABLE) == 0 )
>> +    {
>> +        /*
>> +         * Guest wants to enable INTx. It can't be enabled if:
>> +         *  - host has INTx disabled
>> +         *  - MSI/MSI-X enabled
>> +         */
>> +        if ( pdev->vpci->msi->enabled )
>> +            cmd |= PCI_COMMAND_INTX_DISABLE;
>> +        else
>> +        {
>> +            uint16_t current_cmd = pci_conf_read16(pdev->sbdf, reg);
>> +
>> +            if ( current_cmd & PCI_COMMAND_INTX_DISABLE )
>> +                cmd |= PCI_COMMAND_INTX_DISABLE;
>> +        }
> 
> This last part should be Arm specific. On other architectures we
> likely want the guest to modify INTx disable in order to select the
> interrupt delivery mode for the device.

We cannot allow a guest to clear the bit when it has MSI / MSI-X
enabled - only one of the three is supposed to be active at a time.
(IOW similarly we cannot allow a guest to enable MSI / MSI-X when
the bit is clear.)

Jan