On Mon, Jul 28, 2014 at 07:57:00PM +0100, Konstantin Khlebnikov wrote: > On Mon, Jul 28, 2014 at 10:41 PM, Will Deacon <[email protected]> wrote: > > On Mon, Jul 28, 2014 at 07:25:14PM +0100, Konstantin Khlebnikov wrote: > >> On Mon, Jul 28, 2014 at 10:14 PM, Will Deacon <[email protected]> wrote: > >> > On Tue, Jul 22, 2014 at 04:36:35PM +0100, Konstantin Khlebnikov wrote: > >> >> idmap layout combines both phisical and virtual addresses. > >> >> Everything works fine if ram physically lays below PAGE_OFFSET. > >> >> Otherwise idmap starts punching huge holes in virtual memory layout. > >> >> It maps ram by 2MiB sections, but when it allocates new pmd page it > >> >> cuts 1GiB at once. > >> >> > >> >> This patch makes a copy of all affected pmds from init_mm. > >> >> Only few (usually one) 2MiB sections will be lost. > >> >> This is not eliminates problem but makes it 512 times less likely. > >> > > >> > I'm struggling to understand your commit message, but making a problem > >> > `512 > >> > times less likely' does sound like a bit of a hack to me. Can't we fix > >> > this > >> > properly instead? > >> > >> Yep, my comment sucks. > >> > >> Usually idmap looks like this: > >> > >> |0x00000000 -- <chunk of physical memory in identical mapping > --- | > >> TASK_SIZE -- <kernel space vm layoyt> --- 0xFFFFFFFF | > >> > >> But when that physical memory chunk starts from 0xE8000000 or even > >> 0xF2000000 evenything becomes very complicated. > > > > Why? As long as we don't clobber the kernel text (which would require > > PHYS_OFFSET to be at a really weird alignment and very close to > > PAGE_OFFSET), then you should be alright. Sure, you'll lose things like your > > stack and the vmalloc area etc, but you're running in the idmap, so don't > > use those things. > > Yep, we have piece of hardware with really weird aligned PHYS_OFFSET, > mostly all ram is above 4gb and we was lucky enough to get into the trouble. > > It seems keystone has all memory above 4gb but small piece is mapped below > especially for booting. I suppose it's below PAGE_OFFSET so Cyril > hadn't seen that problem.
Sure, I remember when the keystone support was merged. There's a low alias of memory which isn't I/O coherent, so when we enable the MMU we rewrite the page tables to use the high alias (which is also broken, as I pointed out on the list today). > Also I seen comment somewhere in the code which tells that idrmap pgd is > always below 4gb which isn't quite true. Moreover, I had some experiments with > mapping ram to random places in qemu and seen that kernel cannot boot if > PHYS_OFFSET isn't alligned to 128mb which is strange. > So, it seems there is plenty bugs anound. > > > > > soft_restart is an example of code that deals with these issues. Which code > > is causing you problems? > > That was booting of secondary cpus, all of them. Ok. I think we need more specifics to progress with this patch. It's not clear to me what's going wrong or why your platform is causing the issues you're seeing. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
