Date: Fri, 1 Mar 2013 16:32:10 +0100 From: Olivier Houchard <cognet@ci0.org> To: Svatopluk Kraus <onwahe@gmail.com> Cc: Konstantin Belousov <kostikbel@gmail.com>, freebsd-current@freebsd.org, Olivier Houchard <cognet@ci0.org>, Andrew Turner <andrew@fubar.geek.nz> Subject: Re: ARM pcpu changes, was Re: [patch] i386 pmap sysmaps_pcpu[] atomic access Message-ID: <20130301153210.GA24802@ci0.org> In-Reply-To: <CAFHCsPWWLkGwNJLcC2iibF0if1rEA%2BC=aeM%2B226X3jjt4xE-uw@mail.gmail.com> References: <CAFHCsPUVTM9jfrnzY72YsPszLWkg-UaJcycTR4xXcS%2BfPzS1Vg@mail.gmail.com> <201302201022.29294.jhb@freebsd.org> <CAFHCsPXEDiaKSkHNbycs%2B1DKkL7BcYJBk5eN%2BKpsLY8B%2B-u5gQ@mail.gmail.com> <201302211043.49906.jhb@freebsd.org> <20130225190920.1b7d348d@bender> <20130225124747.GA21027@ci0.org> <20130226210651.580c46a7@bender> <20130226123904.GO2454@kib.kiev.ua> <CAFHCsPWWLkGwNJLcC2iibF0if1rEA%2BC=aeM%2B226X3jjt4xE-uw@mail.gmail.com>
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On Fri, Mar 01, 2013 at 02:20:16PM +0100, Svatopluk Kraus wrote: > On Tue, Feb 26, 2013 at 1:39 PM, Konstantin Belousov > <kostikbel@gmail.com> wrote: > > On Tue, Feb 26, 2013 at 09:06:51PM +1300, Andrew Turner wrote: > >> Does anyone know if it is only curthread that needs to be atomic? If so > >> this should work. Reading the cpuid from the system currently is a > >> single instruction, however it appears the code will need to be reworked > >> for use with multiple levels of affinity, for example when there > >> are clusters of cores the current code will number two cores on > >> different clusters with the same id. > >> > >> I don't think we need to use ldrex/strex to read/write the values, my > >> understanding is the rest should be safe to access without atomic > >> functions. > > > > I read about ARM architecture some time ago. What I am saying below is not > > a proposal, but rather a way for me to learn more about the architecture. > > > > From my reading of the docs, ARM has the shadow registers, in particular, > > the r13 (stack pointer) is shadowed for all privileged modes. Is the shadow > > used by our kernel, is it correctly restored on the context switch ? > > > > If yes, you can easily recover the pcb address from the current sp, > > without accessing any coprocessor registers, and even without any > > assignment of the global register for curthread (which needs to be > > done at the kernel entry). Just align the stack > > start on the 2*PAGE_SIZE boundary (like it is already done for MIPS), > > and do the mask operation on the sp value to get the end of pcb. > > This is atomic and context-switch safe. > > > > pcb us already per-thread, and can store the thread pointer. > > More, you can store the curcpu or cpuid pointer into pcb too, > > and update it on the context switch. > > > > amd64 has an architecturally-defined special register (k)gsbase, which > > effectively must be correctly initialized at each kernel entry, and > > restored on the return to usermode. Since the initialization on entry > > and restoration on exit is not atomic wrt the entry/exit itself, amd64 > > periodically gets a bugs which cause kernel running with user gsbase. > > This is the fatal bug, destroying the kernel structures and allowing the > > CPU privilege mode switch for normal user. > > > > Using the shadow registers for this purpose eliminate the whole source > > of the bugs. > > Well, IMHO, the both kernel and user thread stacks must correctly be > set for current threads, otherwise it doesn't work. So, the idea to > save things on a thread stack and update them on a context switch will > work in general. However, the stack must be aligned to its size. And > this is the price. I have no idea how this kernel stack alignment can > impact kernel virtual space fragmentation. > OTOH, as you say, this kernel stack storage can be used for many things. > Hi, FWIW, I have a proof-of-ooncept patch that does just that, and it seems to work well. Regards, Olivier
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