Date: Wed, 6 Mar 2013 12:33:11 -0700 From: Warner Losh <imp@bsdimp.com> To: Ian Lepore <ian@FreeBSD.org> Cc: freebsd-arm@FreeBSD.org Subject: Re: GENERIC kernel issues Message-ID: <DDB2A45D-33E4-4612-A17B-62F57EBCDFD0@bsdimp.com> In-Reply-To: <1362594744.1291.132.camel@revolution.hippie.lan> 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>
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On Mar 6, 2013, at 11:32 AM, Ian Lepore wrote: > 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). 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... > 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). But that doesn't matter for the kernel so much... > 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. OK. That part makes perfect sense now. Warner=
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