Re: [PATCH v6 08/12] Handle async PF in a guest.

[Avi Kivity <avi@redhat.com>]

  On 10/10/2010 09:56 AM, Gleb Natapov wrote:
> On Sat, Oct 09, 2010 at 08:48:15PM +0200, Avi Kivity wrote:
> >   On 10/07/2010 08:03 PM, Gleb Natapov wrote:
> >  >>   >>
> >  >>   >Host side keeps track of outstanding apfs and will not send apf for the
> >  >>   >same phys address twice. It will halt vcpu instead.
> >  >>
> >  >>   What about different pages, running the scheduler code?
> >  >>
> >  >We can get couple of nested apfs, just like we can get nested
> >  >interrupts. Since scheduler disables preemption second apf will halt.
> >
> >  How much is a couple?
> >
> >  Consider:
> >
> >  SIGSTOP
> >  Entire process swapped out
> >  SIGCONT
> >
> >  We can get APF's on the current code, the scheduler code, the stack,
> >  any debugging code in between (e.g. ftrace), and the page tables for
> >  all of these.
> >
> Lets count them all. Suppose guest is in a userspace process code and
> guest memory is completely swapped out. Guest starts to run and faults
> in userspace. Apf is queued but can't be delivered due to faults in
> idt and exception stack. All of them will be taken synchronously due
> to event pending check. After apf is delivered any fault in apf code
> will be takes synchronously since interrupt are disabled. Just before
> calling schedule() interrupts are enabled, so next pf that will happen
> during call to schedule() will be taken asynchronously. Which will cause
> another call to schedule() at which point vcpu will be halted since two
> apfs happened at the same address. So I counted two of them.
>

Ok.  Feels weird, but I guess this is fine.

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