Re: [Xen-devel] [PATCH V2] x86/vm_event: block interrupt injection for sync vm_events

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 12/11/18 12:14 PM, Roger Pau Monné wrote:
> On Tue, Dec 11, 2018 at 12:01:53PM +0200, Razvan Cojocaru wrote:
>> On 12/10/18 6:59 PM, Razvan Cojocaru wrote:
>>> On 12/10/18 6:49 PM, Roger Pau Monné wrote:
>>>> On Mon, Dec 10, 2018 at 06:01:49PM +0200, Razvan Cojocaru wrote:
>>>>> diff --git a/xen/include/asm-arm/vm_event.h 
>>>>> b/xen/include/asm-arm/vm_event.h
>>>>> index 66f2474..b63249e 100644
>>>>> --- a/xen/include/asm-arm/vm_event.h
>>>>> +++ b/xen/include/asm-arm/vm_event.h
>>>>> @@ -52,4 +52,10 @@ void vm_event_emulate_check(struct vcpu *v, 
>>>>> vm_event_response_t *rsp)
>>>>>      /* Not supported on ARM. */
>>>>>  }
>>>>>  
>>>>> +static inline
>>>>> +void vm_event_block_interrupts(struct vcpu *v, bool value)
>>>>> +{
>>>>> +    /* Not supported on ARM. */
>>>>
>>>> ASSERT_UNREACHABLE?
>>>
>>> Will do (although if you look at the rest of the function in that header
>>> it'll break what appears to be the prior convention).
>>
>> Sorry, on second thought we can't do that, because that function is
>> being called from the common code - which is why the function became
>> necessary. Specifically, this it unconditionally called in
>> monitor_traps(), which is used for all events (ARM and otherwise).
>>
>> So it's valid to call monitor_traps() for ARM vm_events and expect it to
>> run without issue, which ASSERT_UNREACHABLE() would of course break.
> 
> But then the functionality that makes use of vm_event_block_interrupts
> cannot work reliably on ARM and should not be used?

Well, it's currently a no-op on ARM so it doesn't make anything worse.
I don't have access to ARM hardware and am unfamiliar with the specifics
of handling interrupts on ARM with regard to vm_events (or even if this
specific problem applies to ARM) - so it's the best that I am able to do
at the moment.

Of course, this patch can be the basis of a future one for ARM if that
work makes sense (perhaps Tamas has more to say about this), or if an
ARM maintaner can point out what modifications should be done I can
compile-test for ARM with a cross-compiler, _hope_ it works, and
re-submit the patch.


Thanks,
Razvan

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