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SubjectRe: kvm vmload/vmsave vs tss.ist

* Avi Kivity <avi@redhat.com> wrote:

> 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.

agreed, something like this would work too. My suggestion, to eliminate
irqcount altogether and just check RSP against the known-irqstack-range,
could result in slightly smaller (and thus faster) code, but it's a
marginal difference at best.

Ingo


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