From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org (bilbo.ozlabs.org [103.22.144.67]) (using TLSv1.2 with cipher ADH-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3yVq341SWDzDrJw for ; Mon, 6 Nov 2017 21:45:36 +1100 (AEDT) From: Michael Ellerman To: Alexey Kardashevskiy , David Gibson Cc: linuxppc-dev@lists.ozlabs.org, Jonas Pfefferle1 , Nicholas Piggin Subject: Re: [PATCH kernel] powerpc/powernv/ioda: Relax max DMA window size check In-Reply-To: References: <20171031040427.10909-1-aik@ozlabs.ru> Date: Mon, 06 Nov 2017 21:45:34 +1100 Message-ID: <877ev3bs8h.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Alexey Kardashevskiy 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