Re: [Xen-devel] Re: [patch 17/24] Xen-paravirt_ops: avoid havinga bad selector in %gs during context switch

From: Zachary Amsden
Date: Wed Feb 21 2007 - 18:25:51 EST

Andi Kleen wrote:
+ * Temporary hack: zero gs now that we've saved it so that Xen
+ * doesn't try to reload the old value after changing the GDT
+ * during the context switch. This can go away once Xen has
+ * been taught to only reload %gs when it absolutely must.
+ */
+ loadsegment(gs, 0);

Sorry, but i don't really want that unconditionally in the context switch.
Adding a paravirt ops for it would be also ugly. Can Xen be fixed?

I agree with that, but especially because this is not even the right place to save and clear gs; when userspace uses an LDT based %gs, you need to do this all the way back in mmu_context.h before you switch the LDT out. We have the same bug, but I was loathe to try to fix it yet, since it could mean some strange code movement. The awkward bit is that switch_mm doesn't have the pointer to the previous task, only it's mm, so there is no place to save gs. Perhaps it is time to move the LDT switch into switch_to instead of switch_mm? It's not like there are different per-process LDTs in the fixmap or something, which I could see as a valid reason to associate this with the mm switch. But all process LDTs are global, so there isn't an issue.

And I noticed this martian hanging out in mmu_context.h:

#define deactivate_mm(tsk, mm) \
asm("movl %0,%%gs": :"r" (0));

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