Re: [Qemu-devel] [PATCH] vfio/pci-quirks: Only quirk to size of PCI config space

[Laszlo Ersek <lersek@redhat.com>]

On 11/18/15 00:41, Alex Williamson wrote:
> For quirks that support the full PCIe extended config space, limit the
> quirk to only the size of config space available through vfio.  This
> allows host systems with broken MMCONFIG regions to still make use of
> these quirks without generating bad address faults trying to access
> beyond the end of config space exposed through vfio.  This may expose
> direct access to parts of extended config space that we'd prefer not
> to expose, but that's why we have an IOMMU.
> 
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> Reported-by: Ronnie Swanink <ronnie@ronnieswanink.nl>
> ---
>  hw/vfio/pci-quirks.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/vfio/pci-quirks.c b/hw/vfio/pci-quirks.c
> index 30c68a1..e117c41 100644
> --- a/hw/vfio/pci-quirks.c
> +++ b/hw/vfio/pci-quirks.c
> @@ -328,7 +328,7 @@ static void vfio_probe_ati_bar4_quirk(VFIOPCIDevice *vdev, int nr)
>      window->data_offset = 4;
>      window->nr_matches = 1;
>      window->matches[0].match = 0x4000;
> -    window->matches[0].mask = PCIE_CONFIG_SPACE_SIZE - 1;
> +    window->matches[0].mask = vdev->config_size - 1;
>      window->bar = nr;
>      window->addr_mem = &quirk->mem[0];
>      window->data_mem = &quirk->mem[1];
> @@ -674,7 +674,7 @@ static void vfio_probe_nvidia_bar5_quirk(VFIOPCIDevice *vdev, int nr)
>      window->matches[0].match = 0x1800;
>      window->matches[0].mask = PCI_CONFIG_SPACE_SIZE - 1;
>      window->matches[1].match = 0x88000;
> -    window->matches[1].mask = PCIE_CONFIG_SPACE_SIZE - 1;
> +    window->matches[1].mask = vdev->config_size - 1;
>      window->bar = nr;
>      window->addr_mem = bar5->addr_mem = &quirk->mem[0];
>      window->data_mem = bar5->data_mem = &quirk->mem[1];
> @@ -765,7 +765,7 @@ static void vfio_probe_nvidia_bar0_quirk(VFIOPCIDevice *vdev, int nr)
>      memory_region_init_io(mirror->mem, OBJECT(vdev),
>                            &vfio_nvidia_mirror_quirk, mirror,
>                            "vfio-nvidia-bar0-88000-mirror-quirk",
> -                          PCIE_CONFIG_SPACE_SIZE);
> +                          vdev->config_size);
>      memory_region_add_subregion_overlap(&vdev->bars[nr].region.mem,
>                                          mirror->offset, mirror->mem, 1);
>  
> 
> 

$ git log -- hw/vfio/pci-quirks.c | grep Reviewed-by
<nothing>

Okay, I guess my review won't be deemed immediately unnecessary then. :)

(1) Please add to the commit message:

Link: https://www.redhat.com/archives/vfio-users/2015-November/msg00192.html

With that reference:

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

(2) Also, one question just to make sure I understand right: the last
sentence of the commit message means that the left-inclusive,
right-exclusive config space offset range

  [vdev->config_size, PCIE_CONFIG_SPACE_SIZE)

is now directly available to the guest, without QEMU noticing; but,
we're not worried about that, because the guest can't abuse that freedom
anyway for doing arbitrary DMA. Is that right?

If so, then I propose another update to the commit message (replacing
the last sentence):

    This may grant the guest direct access to trailing parts of
    extended config space that we'd prefer not to expose, but that's
    why we have an IOMMU (the guest can't abuse said config space access
    for arbitrary DMA).

Perhaps the above is trivial for you (assuming it is correct at all...);
it may not be trivial for others.

Thanks!
Laszlo