Re: [PATCH v4 for-4.14] x86/monitor: revert default behavior when monitoring register write events

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.06.2020 14:52, Tamas K Lengyel wrote:
> For the last couple years we have received numerous reports from users of
> monitor vm_events of spurious guest crashes when using events. In particular,
> it has observed that the problem occurs when vm_events are being disabled. The
> nature of the guest crash varied widely and has only occured occasionally. 
> This
> made debugging the issue particularly hard. We had discussions about this 
> issue
> even here on the xen-devel mailinglist with no luck figuring it out.
> 
> The bug has now been identified as a race-condition between register event
> handling and disabling the monitor vm_event interface. The default behavior
> regarding emulation of register write events is changed so that they get
> postponed until the corresponding vm_event handler decides whether to allow 
> such
> write to take place. Unfortunately this can only be implemented by performing 
> the
> deny/allow step when the vCPU gets scheduled.
> 
> Due to that postponed emulation of the event if the user decides to pause the
> VM in the vm_event handler and then disable events, the entire emulation step
> is skipped the next time the vCPU is resumed. Even if the user doesn't pause
> during the vm_event handling but exits immediately and disables vm_event, the
> situation becomes racey as disabling vm_event may succeed before the guest's
> vCPUs get scheduled with the pending emulation task. This has been 
> particularly
> the case with VMS that have several vCPUs as after the VM is unpaused it may
> actually take a long time before all vCPUs get scheduled.
> 
> In this patch we are reverting the default behavior to always perform 
> emulation
> of register write events when the event occurs. To postpone them can be turned
> on as an option. In that case the user of the interface still has to take care
> of only disabling the interface when its safe as it remains buggy.
> 
> Fixes: 96760e2fba10 ('vm_event: deny register writes if refused by vm_event
> reply').
> 
> Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>

Applicable parts
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan