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

[Tim Deegan <tim@xxxxxxx>]

At 16:49 +0100 on 26 Aug (1409068141), Jan Beulich wrote:
> >>> On 26.08.14 at 16:56, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> > 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.
> 
> I'd certainly want to hear Tim's opinion here before settling on
> either model.

Yes, I think it's much better to have this be a per-vcpu operation; I
see that's the way it's going already in later versions.

Tim.

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