Re: [Xen-devel] [PATCH RFC V7 4/5] xen, libxc: Request page fault injection via libxc

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 08/26/2014 05:44 PM, Jan Beulich wrote:
>>>> On 26.08.14 at 16:24, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>> On 08/26/2014 05:13 PM, Jan Beulich wrote:
>>>>>> On 13.08.14 at 17:28, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>>> --- a/xen/include/asm-x86/hvm/domain.h
>>>> +++ b/xen/include/asm-x86/hvm/domain.h
>>>> @@ -141,6 +141,14 @@ struct hvm_domain {
>>>>       */
>>>>      uint64_t sync_tsc;
>>>>  
>>>> +    /* Memory introspection page fault injection data. */
>>>> +    struct {
>>>> +        uint64_t address_space;
>>>> +        uint64_t virtual_address;
>>>> +        uint32_t errcode;
>>>> +        bool_t valid;
>>>> +    } fault_info;
>>>
>>> Sorry for noticing this only now, but how can this be a per-domain
>>> thing rather than a per-vCPU one?
>>
>> The requirement for our introspection application has simply been to
>> bring back in a swapped-out page, regardless of what VCPU ends up
>> actually doing it.
> 
> But please remember that what you add to the public code base
> shouldn't be tied to specific needs of your application, it should
> be coded in a generally useful way.

Of course, perhaps I should have written "the scenario we're working
with" rather than "the requirement for our application". I'm just trying
to understand all the usual cases for this.

> Furthermore, how would this work if you have 2 vCPU-s hit such
> a condition, and you need to bring in 2 pages in parallel?

Since this is all happening in the context of processing mem_events,
it's not really possible for two VCPUs to need to do this in parallel,
since processing mem_events is being done sequentially. A VCPU needs to
put a mem_event in the ring buffer and pause before this hypercall can
be called from userspace.


Thanks,
Razvan Cojocaru

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