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

Kai Huang
2014-01-19T14:03:59Z

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?

Thanks,
-Kai
> size = start < bar->size ? bar->size - start : 0;
> strncat(name, " msix-hi", sizeof(name) - strlen(name) - 1);
>
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