[PATCH kernel] powerpc/powernv/ioda: Relax max DMA window size check

[Jonas Pfefferle1]

Michael Ellerman <mpe at ellerman.id.au> wrote on 11/06/2017 11:45:34 AM:

> From: Michael Ellerman <mpe at ellerman.id.au>
> To: Alexey Kardashevskiy <aik at ozlabs.ru>, David Gibson
> <david at gibson.dropbear.id.au>
> Cc: linuxppc-dev at lists.ozlabs.org, Jonas Pfefferle1
> <JPF at zurich.ibm.com>, Nicholas Piggin <npiggin at gmail.com>
> Date: 11/06/2017 11:45 AM
> Subject: Re: [PATCH kernel] powerpc/powernv/ioda: Relax max DMA
> window size check
>
> Alexey Kardashevskiy <aik at ozlabs.ru> writes:
>
> > On 31/10/17 15:04, Alexey Kardashevskiy wrote:
> >> DMA windows can only have a size of power of two on IODA2 hardware and
> >> using memory_hotplug_max() to determine the upper limit won't work
> >> correcly if it returns not power of two value.
> >>
> >> This relaxes the check by rounding up the value returned by
> >> memory_hotplug_max().
> >>
> >> It is expected to impact DPDK on machines with non-power-of-two RAM
size,
> >> mostly. KVM guests are less likely to be affected as usually guests
get
> >> less than half of hosts RAM.
> >
> >
> > It was pointed out that this check is quite useless anyway as the
vm_locked
> > memory limit should hit first, and if that is not set or the user got
the
> > root privilege level, then there are easier ways to crash the host so I
am
> > thinking of:
> >
> >
> > diff --git a/arch/powerpc/platforms/powernv/pci-ioda.c
> > b/arch/powerpc/platforms/powernv/pci-ioda.c
> > index 269f119e4b3c..a47e4cf343b2 100644
> > --- a/arch/powerpc/platforms/powernv/pci-ioda.c
> > +++ b/arch/powerpc/platforms/powernv/pci-ioda.c
> > @@ -2769,7 +2769,7 @@ static long pnv_pci_ioda2_table_alloc_pages(int
nid,
> > __u64 bus_offset,
> >         if (!levels || (levels > POWERNV_IOMMU_MAX_LEVELS))
> >                 return -EINVAL;
> >
> > -       if ((window_size > memory_hotplug_max()) ||
> > !is_power_of_2(window_size))
> > +       if (!is_power_of_2(window_size))
> >                 return -EINVAL;
> >
> >
> >
> > Makes sense?
>
> Sounds reasonable.
>
> Execpt where is the vm_locked check? I think it's in the VFIO driver? If
> so I guess the only concern is that this code might be called via some
> other path that doesn't do that check.
>
> cheers
>

The vm_locked is incremented here:
http://elixir.free-electrons.com/linux/v4.13.11/source/drivers/vfio/vfio_iommu_spapr_tce.c#L176
resp.
http://elixir.free-electrons.com/linux/v4.13.11/source/arch/powerpc/mm/mmu_context_iommu.c#L124
on VFIO_IOMMU_SPAPR_REGISTER_MEMORY. From my understanding only pages
that have been registered through here can be mapped with MAP_DMA.

Cheers,
Jonas
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