Date: Mon, 5 May 2008 09:50:57 -0400 From: John Baldwin <jhb@freebsd.org> To: Juergen Lock <nox@jelal.kn-bremen.de> Cc: freebsd-emulation@freebsd.org, freebsd-amd64@freebsd.org Subject: Re: seems I finally found what upset kqemu on amd64 SMP... shared gdt! (please test patch :) Message-ID: <200805050950.57484.jhb@freebsd.org> In-Reply-To: <20080503131139.GA37131@saturn.kn-bremen.de> References: <20080429222458.GA20855@saturn.kn-bremen.de> <200805011335.06415.jhb@freebsd.org> <20080503131139.GA37131@saturn.kn-bremen.de>
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On Saturday 03 May 2008 09:11:39 am Juergen Lock wrote: > On Thu, May 01, 2008 at 01:35:06PM -0400, John Baldwin wrote: > > On Thursday 01 May 2008 11:53:04 am Juergen Lock wrote: > > > On Thu, May 01, 2008 at 10:11:06AM -0400, John Baldwin wrote: > > > > On Thursday 01 May 2008 06:19:51 am Juergen Lock wrote: > > > > > On Wed, Apr 30, 2008 at 12:24:58AM +0200, Juergen Lock wrote: > > > > > > Yeah, the amd64 kernel reuses the same gdt to setup all cpus, > > > > > > causing kqemu to end up restoring the interrupt stackpointer > > > > > > (after running guest code using its own cpu state) from the tss > > > > > > of the last cpu, regardless which cpu it happened to run on. And > > > > > > that then causes the last cpu's (usually) idle thread's stack to > > > > > > get smashed and the host doing multiple panics... (Which also > > > > > > explains why pinning qemu onto > > > > cpu > > > > > > > > 1 worked on a 2-way host.) > > > > > > > > > > Hmm maybe the following is a little more clear: kqemu sets up its > > > > > own cpu state and has to save and restore the original state > > > > > because of > > > > that, > > > > > > > so among other things it does an str insn (store task register), > > > > > and > > > > later > > > > > > > an ltr insn (load task register) using the value it got from the > > > > > first str insn. That ltr insn loads the selector for the tss which > > > > > is stored in the gdt, and that entry in the gdt is different for > > > > > each cpu, but > > > > since > > > > > > > a single gdt was reused to setup the cpus at boot (in > > > > > init_secondary() > > > > in > > > > > > > /sys/amd64/amd64/mp_machdep.c), it still points to the tss for the > > > > > last cpu, instead of to the right one for the cpu the ltr insn gets > > > > > executed > > > > on. > > > > > > > That is what the kqemu_tss_workaround() in the patch `fixes'... > > > > > > > > Perhaps kqemu shouldn't be doing str/ltr on amd64 instead? The > > > > things > > > > i386 > > > > > > uses a separate tss for in the kernel (separate stack for double > > > > faults) > > > > is > > > > > > handled differently on amd64 (on amd64 we make the double fault > > > > handler > > > > use > > > > > > one of the IST stacks). > > > > > > Well, kqemu uses its own gdt, tss and everything while running guest > > > code in its monitor, so it kinda has to do the str/ltr.s to setup its > > > stuff, run guest code, and then restore the original state of things. > > > (And `restore original state of things' is what failed here.) > > > > > > Oh and also the tss does seem to be used for the interrupt stack on > > > amd64 too, at least thats the one that ended up wrong and caused the > > > panics I saw... > > > > The single TSS holds the IST pointers. On i386 we use a separate TSS for > > double faults, but on amd64 a double fault uses the same TSS but uses the > > IST pointers from that same TSS. The TSS also holds the ring stack > > pointer for when syscalls, interrupts, and traps from userland cross from > > ring 3 to ring 0 which is probably why you got a panic. > > Yeah thats where it happened. > > > Because of the fact that amd64 in normal operation never changes the task > > register (and that the gdt isn't used quite the same either, all the > > per-cpu stuff is via FSBASE and GSBASE) I don't expect the kernel to > > change to use a per-cpu gdt or the like. I think you will need to use > > the current approach of patching kqemu to fixup the tss/gdt when > > reloading the task register. You might want to make it a regular part of > > the code rather than a workaround as a result. > > Hmm okay, how would you call it then, kqemu_tss_fixup? Sure. -- John Baldwin
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