From mboxrd@z Thu Jan 1 00:00:00 1970 From: stuart.yoder@freescale.com (Stuart Yoder) Date: Thu, 30 Jul 2015 18:10:33 +0000 Subject: [RFC] arm64: defconfig: enable 48-bit VA by default In-Reply-To: References: <55B0E1AD.60409@arm.com> <20150730101358.GA30796@e104818-lin.cambridge.arm.com> <20150730161210.GE407@e104818-lin.cambridge.arm.com> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org > -----Original Message----- > From: Ard Biesheuvel [mailto:ard.biesheuvel at linaro.org] > Sent: Thursday, July 30, 2015 12:46 PM > To: Yoder Stuart-B08248 > Cc: Catalin Marinas; Mark Rutland; Marc Zyngier; Will Deacon; Newton Peter-RA3823; linux-arm- > kernel at lists.infradead.org > Subject: Re: [RFC] arm64: defconfig: enable 48-bit VA by default > > On 30 July 2015 at 18:32, Stuart Yoder wrote: > > > > > >> -----Original Message----- > >> From: Catalin Marinas [mailto:catalin.marinas at arm.com] > >> > >> For the time being, I would keep defconfig to 39-bit VA. If we start > >> seeing systems with lots of RAM (over 256GB), we'll probably change the > >> defconfig. > > > > Somone who follows the "Principles" doc "correctly" will hit an issue > > with the linear memory map after 32GB of RAM, right? 2 GB in 1st alias, > > 30 GB in 2nd alias, and rest at the 544 GB offset. > > > > But, I guess it still remains to be seen who else will hit that > > and when. > > > > Hi Stuart, > > I think you may be overestimating the significance of defconfig. I might be, it's just a paranoia point...which led to wondering about what to expect with respect 48-bit VA and 64K pages going forward. > As you pointed out, distros have their own configs (and many out of tree > patches) for the binary kernels they ship. Fedora currently ships with > 64k pages because it was the only way to support Seattle before the > 4-level 4kb pages implementation was introduced. They will likely keep > doing that, or switch to 4-level 4 kB pages since, as you pointed out, > it is fairly likely that servers have more than 32 GB DRAM, which > means that they need more than 256 GB of virtual address space in the > linear region if they follow the ARM recommendation. > > Whether defconfig supports your platform optimally has nothing to do > with that. Of course, we should deal with the unexpected memory layout > gracefully, which is why Mark Rutland and myself proposed patches to > fix the panic you reported. But in a development context, I think it > is perfectly acceptable to simply load the kernel at 0x80_8000_0000, > and be able to run defconfig fine while losing just 2 GB of your 16 GB > at the low end. Of course, you would never ship a system like that, > but that is not what defconfig is meant to cater for. Thanks for the patch and all your input, the picture and options are quite a bit clearer to me now. Thanks, Stuart