Re: [Qemu-devel] [PATCH qemu RFC 1/7] vfio/spapr: Fix indirect levels calculation

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Nov 13, 2018 at 07:30:58PM +1100, Alexey Kardashevskiy wrote:
> The current code assumes that we can address more bits on a PCI bus
> for DMA than we really can. Limit to the known tested maximum of 55 bits
> and assume 64K IOMMU pages.

This doesn't make much sense to me.  It looks nothing like the
calculation it's replacing, and not just for extreme values.  For
example the new calculation doesn't depend on the size of the window
at all, whereas the previous one did.  You say you're assuming 64k
IOMMU pages, but create.page_shift (the actual IOMMU page size) is in
there.  Nor is it clear to me why the maximum PCI address is relevant
to the number of levels in the first place.

In addition, AFAICT the new calculation gives the answer '2' for all
likely IOMMU page sizes (4k, 64k, 2M)

> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
>  hw/vfio/spapr.c      | 3 ++-
>  hw/vfio/trace-events | 2 +-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/vfio/spapr.c b/hw/vfio/spapr.c
> index becf71a..f5fdc53 100644
> --- a/hw/vfio/spapr.c
> +++ b/hw/vfio/spapr.c
> @@ -183,7 +183,7 @@ int vfio_spapr_create_window(VFIOContainer *container,
>      entries = create.window_size >> create.page_shift;
>      pages = MAX((entries * sizeof(uint64_t)) / getpagesize(), 1);
>      pages = MAX(pow2ceil(pages), 1); /* Round up */
> -    create.levels = ctz64(pages) / 6 + 1;
> +    create.levels = MAX(1, (55 - create.page_shift) / 16);
>  
>      ret = ioctl(container->fd, VFIO_IOMMU_SPAPR_TCE_CREATE, &create);
>      if (ret) {
> @@ -200,6 +200,7 @@ int vfio_spapr_create_window(VFIOContainer *container,
>          return -EINVAL;
>      }
>      trace_vfio_spapr_create_window(create.page_shift,
> +                                   create.levels,
>                                     create.window_size,
>                                     create.start_addr);
>      *pgsize = pagesize;
> diff --git a/hw/vfio/trace-events b/hw/vfio/trace-events
> index a85e866..db730f3 100644
> --- a/hw/vfio/trace-events
> +++ b/hw/vfio/trace-events
> @@ -128,6 +128,6 @@ vfio_prereg_listener_region_add_skip(uint64_t start, uint64_t end) "0x%"PRIx64"
>  vfio_prereg_listener_region_del_skip(uint64_t start, uint64_t end) "0x%"PRIx64" - 0x%"PRIx64
>  vfio_prereg_register(uint64_t va, uint64_t size, int ret) "va=0x%"PRIx64" size=0x%"PRIx64" ret=%d"
>  vfio_prereg_unregister(uint64_t va, uint64_t size, int ret) "va=0x%"PRIx64" size=0x%"PRIx64" ret=%d"
> -vfio_spapr_create_window(int ps, uint64_t ws, uint64_t off) "pageshift=0x%x winsize=0x%"PRIx64" offset=0x%"PRIx64
> +vfio_spapr_create_window(int ps, unsigned int levels, uint64_t ws, uint64_t off) "pageshift=0x%x levels=%u winsize=0x%"PRIx64" offset=0x%"PRIx64
>  vfio_spapr_remove_window(uint64_t off) "offset=0x%"PRIx64
>  vfio_spapr_group_attach(int groupfd, int tablefd) "Attached groupfd %d to liobn fd %d"

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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