Re: [PATCH 3/3] VFIO: Direct access config reg without capability

Alex Williamson
2013-03-21T00:58:41Z

On Fri, 2013-03-15 at 15:26 +0800, Gavin Shan wrote:
> The config registers in [0, 0x40] is being supported by VFIO. Apart
> from that, the other config registers should be coverred by PCI or
> PCIe capability. However, there might have some PCI devices (be2net)
> who has config registers (0x7c) out of [0, 0x40], and don't have
> corresponding PCI or PCIe capability. VFIO will return 0x0 on reading
> those registers and writing is dropped. It caused the be2net driver
> fails to be loaded because 0x0 returned from its config register 0x7c.
>
> The patch changes the behaviour so that those config registers out
> of [0, 0x40] and don't have corresponding PCI or PCIe capability
> will be accessed directly.
>
> Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
> ---

Hi Gavin,

I'm onboard with making this change now, but this patch isn't
sufficient. The config space map uses a byte per dword to index the
capability since both standard and extended capabilities are dword
aligned. We currently have a bug that this patch exposes that we round
the length down, ex. a 14 byte MSI capability becomes 12 bytes leaving
the message data now exposed and writable with this patch. That bug can
be fixed by aligning the length so the capability fills the dword, but
notice that 0x7c on the be2net is filling one of these gaps. So fixing
that bug attaches that gap to the previous capability instead of
allowing direct access.

So, before we can make this change we need to fix the config map to have
byte granularity. Thanks,

Alex

> drivers/vfio/pci/vfio_pci_config.c | 31 ++++++++++++++++++++-----------
> 1 files changed, 20 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
> index 964ff22..5ea3afb 100644
> --- a/drivers/vfio/pci/vfio_pci_config.c
> +++ b/drivers/vfio/pci/vfio_pci_config.c
> @@ -1471,18 +1471,27 @@ static ssize_t vfio_config_do_rw(struct vfio_pci_device *vdev, char __user *buf,
>
> cap_id = vdev->pci_config_map[*ppos / 4];
>
> + /*
> + * Some PCI device config registers might not be coverred by
> + * capability and useful. We will enable direct access to
> + * those registers.
> + */
> if (cap_id == PCI_CAP_ID_INVALID) {
> - if (iswrite)
> - return ret; /* drop */
> -
> - /*
> - * Per PCI spec 3.0, section 6.1, reads from reserved and
> - * unimplemented registers return 0
> - */
> - if (copy_to_user(buf, &val, count))
> - return -EFAULT;
> -
> - return ret;
> + if (iswrite) {
> + if (copy_from_user(&val, buf, count))
> + return -EFAULT;
> + ret = vfio_user_config_write(vdev->pdev, (int)(*ppos),
> + val, count);
> + return ret ? ret : count;
> + } else {
> + ret = vfio_user_config_read(vdev->pdev, (int)(*ppos),
> + &val, count);
> + if (ret)
> + return ret;
> + if (copy_to_user(buf, &val, count))
> + return -EFAULT;
> + return count;
> + }
> }
>
> /*