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

[Alexey Kardashevskiy]

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?


> 
> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
> ---
>  arch/powerpc/platforms/powernv/pci-ioda.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/platforms/powernv/pci-ioda.c b/arch/powerpc/platforms/powernv/pci-ioda.c
> index 269f119e4b3c..4c62162da181 100644
> --- a/arch/powerpc/platforms/powernv/pci-ioda.c
> +++ b/arch/powerpc/platforms/powernv/pci-ioda.c
> @@ -2769,7 +2769,8 @@ 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 ((window_size > roundup_pow_of_two(memory_hotplug_max())) ||
> +			!is_power_of_2(window_size))
>  		return -EINVAL;
>  
>  	/* Adjust direct table size from window_size and levels */
> 


-- 
Alexey