Re: 2.6.32 PV Xen donU guest panic on nested call to arch_enter_lazy_mmu_mode()

[Chuck Anderson <chuck.anderson@oracle.com>]

Jeremy,
  Is it possible for an ongoing lazy mode update to have batched some 
MMU updates; an interrupt occurs; an interrupt routine does a non-lazy 
MMU update for a PTE that is also in the lazy update queue; that update 
is overwritten on return from the interrupt when the update queue is 
flushed?  Or are the PTE updates protected by a lock?  If they are, 
wouldn't we deadlock in the interrupt routine when it tries to obtain 
that (I assume) spinlock?
Chuck

Jeremy Fitzhardinge wrote:
> Disabling interrupts would cause too much latency.  I think we may have
> done this at one point, but it is very antisocial.
>
> Since lazy mode is effectively disabled in interrupt handlers anyway, it
> should just be enough to ignore enter/leave requests.  Does this work
> for you?
>
> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
> Date: Wed, 8 Dec 2010 14:21:16 -0800
> Subject: [PATCH] x86/paravirt: don't enter/leave lazy mode in interrupts.
>
> We already ignore the current state of lazy mode in interrupts, but we
> should also ignore any attempt to enter/leave lazy mode within
> an interrupt context.
>
> enter_lazy() will BUG if it sees an attempt at a nested entry to lazy
> mode, which is generally an error.  However, it's possible that an
> interrupt handler may do something that would trigger a batched MMU
> update, for example, and that could interrupt an existing batched update.