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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Nov 02, 2021 at 12:19:13PM +0100, Jan Beulich wrote:
> 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.)

Sure, but this code is making the bit sticky, by not allowing
INTX_DISABLE to be cleared once set. We do not want that behavior on
x86, as a guest can decide to use MSI or INTx. The else branch needs
to be Arm only.

Regards, Roger.