Re: [PULL 7/7] vfio: fix mapping of MSIX bar

Kai Huang
2014-01-19T15:46:04Z

On Sun, Jan 19, 2014 at 10:11 PM, Alex Williamson
<alex.williamson@redhat.com> wrote:
> On Sun, 2014-01-19 at 22:03 +0800, Kai Huang wrote:
>> On Sat, Jan 18, 2014 at 3:25 AM, Alex Williamson
>> <alex.williamson@redhat.com> wrote:
>> > From: Alexey Kardashevskiy <aik@ozlabs.ru>
>> >
>> > VFIO virtualizes MSIX table for the guest but not mapping the part of
>> > a BAR which contains an MSIX table. Since vfio_mmap_bar() mmaps chunks
>> > before and after the MSIX table, they have to be aligned to the host
>> > page size which may be TARGET_PAGE_MASK (4K) or 64K in case of PPC64.
>> >
>> > This fixes boundaries calculations to use the real host page size.
>> >
>> > Without the patch, the chunk before MSIX table may overlap with the MSIX
>> > table and mmap will fail in the host kernel. The result will be serious
>> > slowdown as the whole BAR will be emulated by QEMU.
>> >
>> > Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
>> > ---
>> > hw/misc/vfio.c | 6 +++---
>> > 1 file changed, 3 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/hw/misc/vfio.c b/hw/misc/vfio.c
>> > index 432547c..8a1f1a1 100644
>> > --- a/hw/misc/vfio.c
>> > +++ b/hw/misc/vfio.c
>> > @@ -2544,7 +2544,7 @@ static void vfio_map_bar(VFIODevice *vdev, int nr)
>> > * potentially insert a direct-mapped subregion before and after it.
>> > */
>> > if (vdev->msix && vdev->msix->table_bar == nr) {
>> > - size = vdev->msix->table_offset & TARGET_PAGE_MASK;
>> > + size = vdev->msix->table_offset & qemu_host_page_mask;
>> > }
>> >
>> > strncat(name, " mmap", sizeof(name) - strlen(name) - 1);
>> > @@ -2556,8 +2556,8 @@ static void vfio_map_bar(VFIODevice *vdev, int nr)
>> > if (vdev->msix && vdev->msix->table_bar == nr) {
>> > unsigned start;
>> >
>> > - start = TARGET_PAGE_ALIGN(vdev->msix->table_offset +
>> > - (vdev->msix->entries * PCI_MSIX_ENTRY_SIZE));
>> > + start = HOST_PAGE_ALIGN(vdev->msix->table_offset +
>> > + (vdev->msix->entries * PCI_MSIX_ENTRY_SIZE));
>> >
>> Hi Alex,
>>
>> I am new to vfio and qemu, and have some questions. Does MSIX have one
>> dedicated bar when qemu emulating the device? Looks your code maps
>> both the content before and after the MSIX table? If MSIX has
>> dedicated bar, I think we can just skip the MSIX bar, why do we need
>> to map the context before and after the MSIX table?
>
> vfio is used to pass through existing physical devices. We don't get to
> define the MSI-X layout of those devices. Therefore we must be prepared
> to handle any possible layout. The BAR may be dedicated to the MSI-X
> table or it may also include memory mapped register space for the
> device. Thanks,
>
> Alex
>

This sounds reasonable. So if there's bar contains both MSIX table and
register, the register access will be trapped and emulated?

Btw, did vfio_mmap_bar consider the pedding bit array table? I don't
think they can be accessed directly by userspace either.

Thanks,
-Kai