Re: [Xen-devel] [PATCH V3] x86/vm_event: block interrupt injection for sync vm_events

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 1/14/19 4:42 PM, Juergen Gross wrote:
> On 14/01/2019 11:56, Razvan Cojocaru wrote:
>> On 1/14/19 11:53 AM, Jan Beulich wrote:
>>>>>> On 14.01.19 at 10:34, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>>> On 1/12/19 12:04 AM, Boris Ostrovsky wrote:
>>>>> On 12/14/18 6:49 AM, Razvan Cojocaru wrote:
>>>>>> Block interrupts (in vmx_intr_assist()) for the duration of
>>>>>> processing a sync vm_event (similarly to the strategy
>>>>>> currently used for single-stepping). Otherwise, attempting
>>>>>> to emulate an instruction when requested by a vm_event
>>>>>> reply may legitimately need to call e.g.
>>>>>> hvm_inject_page_fault(), which then overwrites the active
>>>>>> interrupt in the VMCS.
>>>>>>
>>>>>> The sync vm_event handling path on x86/VMX is (roughly):
>>>>>> monitor_traps() -> process vm_event -> vmx_intr_assist()
>>>>>> (possibly writing VM_ENTRY_INTR_INFO) ->
>>>>>> hvm_vm_event_do_resume() -> hvm_emulate_one_vm_event()
>>>>>> (possibly overwriting the VM_ENTRY_INTR_INFO value).
>>>>>>
>>>>>> This patch may also be helpful for the future removal
>>>>>> of may_defer in hvm_set_cr{0,3,4} and hvm_set_msr().
>>>>>>
>>>>>> Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
>>>>>
>>>>>
>>>>> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
>>>>
>>>> Thanks! So now we have three reviewed-bys, if I'm not mistaken all we
>>>> need is Tamas' (for the vm_event part) and Julien / Stefano's (for ARM)
>>>> acks (or otherwise).
>>>
>>> And you'd need to talk Jürgen into allowing this in, now that we're
>>> past the freeze point.
>>
>> (Adding Jürgen to the conversation.)
>>
>> Right, that would be ideal if possible - the code has absolutely no
>> impact on anything unless vm_event is active on the domain, which is
>> never the case for the use-cases considered for a Xen release.
>>
>> So the case I'm making for the patch to go in 4.12 is that:
>>
>> 1. It's perfectly harmless (it affects nothing, except for introspection).
>>
>> 2. It's trivial and thus very easy to see that it's correct.
>>
>> 3. It fixes a major headache for us, and thus it is a great improvement
>> from an introspection standpoint (fixes OS crashes / hangs which we'd
>> otherwise need to work around in rather painful ways).
>>
>> 4. V3 of the patch has been sent out on Dec 14th - it's just that
>> reviewers have had other priorities and it did not gather all acks in time.
>>
>> However, if it's not possible or desirable to allow this in the next
>> best thing is to at least have all the acks necessary for it to go in
>> first thing once the freeze is over.
>>
>> From Julien's reply I understand that the last ack necessary is Tamas'.
> 
> With that ack just arrived:
> 
> Release-acked-by: Juergen Gross <jgross@xxxxxxxx>
AFAICT this is fine to apply to staging now, am I incorrect?


Thanks,
Razvan

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