RE: [PATCH 3/3] vfio: Report PASID capability via VFIO_DEVICE_FEATURE ioctl

[Duan, Zhenzhong]

>-----Original Message-----
>From: Alex Williamson <alex.williamson@xxxxxxxxxx>
>Sent: Tuesday, December 12, 2023 2:04 AM
>Subject: Re: [PATCH 3/3] vfio: Report PASID capability via
>VFIO_DEVICE_FEATURE ioctl
>
>On Sun, 26 Nov 2023 22:39:09 -0800
>Yi Liu <yi.l.liu@xxxxxxxxx> wrote:
>
>> This reports the PASID capability data to userspace via
>VFIO_DEVICE_FEATURE,
>> hence userspace could probe PASID capability by it. This is a bit different
>> with other capabilities which are reported to userspace when the user
>reads
>> the device's PCI configuration space. There are two reasons for this.
>>
>>  - First, Qemu by default exposes all available PCI capabilities in vfio-pci
>>    config space to the guest as read-only, so adding PASID capability in the
>>    vfio-pci config space will make it exposed to the guest automatically
>while
>>    an old Qemu doesn't really support it.
>
>Shouldn't we also be working on hiding the PASID capability in QEMU
>ASAP?  This feature only allows QEMU to know PASID control is actually
>available, not the guest.  Maybe we're hoping this is really only used
>by VFs where there's no capability currently exposed to the guest?

PASID capability is not exposed to QEMU through config space,
VFIO_DEVICE_FEATURE ioctl is the only interface to expose PASID
cap to QEMU for both PF and VF.

/*
 * Lengths of PCIe/PCI-X Extended Config Capabilities
 *   0: Removed or masked from the user visible capability list
 *   FF: Variable length
 */
static const u16 pci_ext_cap_length[PCI_EXT_CAP_ID_MAX + 1] = {
...
        [PCI_EXT_CAP_ID_PASID]  =       0,      /* not yet */
}

Thanks
Zhenzhong