Re: kvm vmload/vmsave vs tss.ist

From: Avi Kivity
Date: Thu Dec 25 2008 - 13:17:48 EST


Ingo Molnar wrote:
I think it's enough to switch %rsp before incrementing irqcount, no?

no - that would introduce a small race: if an exception (say an NMI or MCE, or a debug trap) happens in that small window then the exception context thinks that it's on the IRQ stack already, and would use the task stack.


I'm suggesting

check irqcount
if (wasnt_in_irq)
rsp = irqstack
++irqcount

If the NMI happens before the increment, we'll switch the stack unconditionally, and if the NMI happens after the increment, then we won't switch the stack, but we're guaranteed to be on the irqstack anyway. The window size is negative :)

Similarly, the exit path should be

oldstack_reg = oldstack;
--irqcount;
rsp = oldstack_register;

To guarantee that by the time we decrement irqcount, we don't need the stack anymore.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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