Date: Wed, 6 Mar 2013 14:57:56 -0700 From: Warner Losh <imp@bsdimp.com> To: Aleksandr Rybalko <ray@freebsd.org> Cc: freebsd-arm@FreeBSD.org, Ian Lepore <ian@FreeBSD.org> Subject: Re: GENERIC kernel issues Message-ID: <4C0099FA-BBBE-4F93-8C97-CE5B79465829@bsdimp.com> In-Reply-To: <20130306234004.bf113967.ray@freebsd.org> References: <DF7B73D4-BE50-4E75-8D5B-FE19A4764F31@freebsd.org> <1362445777.1195.299.camel@revolution.hippie.lan> <3DFABC9A-876A-4F34-9E15-E4C630D7B077@bsdimp.com> <1362542286.1291.94.camel@revolution.hippie.lan> <4CF23AFE-DD69-40AA-ACFB-46F055F0AA3F@bsdimp.com> <1362594744.1291.132.camel@revolution.hippie.lan> <DDB2A45D-33E4-4612-A17B-62F57EBCDFD0@bsdimp.com> <20130306234004.bf113967.ray@freebsd.org>
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On Mar 6, 2013, at 2:40 PM, Aleksandr Rybalko wrote: > On Wed, 6 Mar 2013 12:33:11 -0700 > Warner Losh <imp@bsdimp.com> wrote: >=20 >>=20 >> On Mar 6, 2013, at 11:32 AM, Ian Lepore wrote: >>=20 >>> On Wed, 2013-03-06 at 10:03 -0700, Warner Losh wrote: >>>> On Mar 5, 2013, at 8:58 PM, Ian Lepore wrote: >>> [...] >>>>> We essentially pull off the same mapping trick with the kernel, >>>>> except that very early in locore.s the code is carefully crafted >>>>> to work right whether on physical or virtual addressing, just >>>>> long enough to get the MMU turned off. Then it sets up page >>>>> tables to map the physical pages the kernel has been loaded into >>>>> to match the virtual addresses it was linked for. All of that >>>>> only works if the low-order bits of the virtual address it was >>>>> linked for match the physical address it was loaded at. That is, >>>>> if linked for 0xC0001000 it must be loaded at 0xNNNN1000 >>>>> physical, where the N bits can be anything. >>>>=20 >>>> Right, but can't we get that from the virtual address. >>>=20 >>> Not always. You can always figure out the right virtual address if >>> you have the physical (you just OR-in 0xC0000000), but you can't >>> always go the other way. If all you know is 0xC0010000 you have no >>> idea whether the underlying physical address might be 0x00010000 or >>> 0x80010000. Our current code that assumes you can do >>> phys=3Dpc&0xf0000000 is wrong for the same reason (but has been >>> working okay by accident). >>=20 >> The phys segment is pc & 0xf0000000 before you turn on the MMU >> (assuming 256MB chip select offsets, adding another F would get that >> down to 16MB chip selects, which is definitely good enough). After we >> MMU start, it isn't, and our code shouldn't do that, unless it is >> followed by oring in the physical segment... >>=20 >>> That's part of why I've been working towards getting our arm >>> ldscript to put proper physical addresses in the elf headers >>> instead of virtual, in the fields that are supposed to have >>> phsyical addresses in them (entry and program-header paddr fields). >>=20 >> But that doesn't matter for the kernel so much... >>=20 >>> With this scheme SoC-specific kernels will be linked with PHYSADDR=3D >>> the real physical address and can be loaded by any standard elf >>> loader because the headers are correct. A generic kernel will be >>> linked with PHYSADDR=3Doffset where offset is just the low-order = part >>> of the address and ubldr can load the kernel into whatever physical >>> memory is available as long as the offset part stays the same. >>=20 >> OK. That part makes perfect sense now. >>=20 >> Warner >> _______________________________________________ >> freebsd-arm@freebsd.org mailing list >> http://lists.freebsd.org/mailman/listinfo/freebsd-arm >> To unsubscribe, send any mail to = "freebsd-arm-unsubscribe@freebsd.org" >=20 > Maybe we should just let ubldr to enable MMU for us? Like we do for > i386, IIRC. One weak reason is that uboot and other loaders that are used to loading = Linux are forbidden from having the MMU enabled when they transfer = control to the kernel they boot. We'd want to still work there, and = would need extra/different code to cope with the booted with the MMU = enabled and booted without the MMU enabled. Warner
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