Re: [Xen-devel] Questions regarding implementing mem_event for PV

["Aravindh Puthiyaparambil (aravindp)" <aravindp@xxxxxxxxx>]

> On Thu, Oct 17, 2013 at 7:59 PM, Aravindh Puthiyaparambil (aravindp)
> <aravindp@xxxxxxxxx> wrote:
> >> > I am looking in to implementing mem_event for PV guests that use PV
> >> mmu_ops. Initially I am thinking of supporting 64-bit PV guests. I think
> only
> >> RW, RX and R permissions would be possible. I am looking at the hypercall
> >> interfaces that do the pagetable manipulations. In addition to the
> hypercalls I
> >> am also looking at the pagetable write emulation code.
> >> >
> >> > My plan is to set the page permissions in:
> >> > do_mmu_update()
> >> > do_mmuext_op(MMUEXT_PIN_L1_TABLE)
> >> > __do_update_va_mapping()
> >> > ptwr_emulated_update()
> >> >
> >>
> >> I'm afraid you can't do that, and still support the PV API as it
> >> stands.  The guest had read access to its pagetables, and it does not
> >> expect Xen to change them underfoot.  E.g. if you temporarily mark a
> >> page as inaccessible and the guest walks its own pagetables to
> >> diagnose a page fault, it could see your change and send SIGSEGV to
> >> the current process.
> >>
> >> So if you want to have permissions in the pagetables that aren't the
> >> ones the guest asked for, your choices are:
> >>
> >>  (a) use shadow pagetables.  This is what the live migration code
> >>      does in order to intercept writes for tracking memory dirtying.
> >>      Slow, but should work for most things.
> >>
> >>  (b) Wait for the PVH patch series to go in (and the equivalent
> >>      series in Linux and FreeBSD kernels).  PVH guests are enough
> >>      like HVM ones in the Xen code that mem_events will be much easier.
> >>
> >> (b) is probably much eaier, if you have the choice.
> >
> > I don't have the choice of waiting for (b) so I guess I would have to go 
> > down
> the harder (a) route. Let me explore that and get back to you with my
> thoughts.
> 
> FWIW, I think a "tech preview" version should go in fairly soon.  At
> the developer meeting we agreed that it would be best to check in what
> we have once I get the bugs worked out, so that people can start
> working on additional functionality (like the mem events) in parallel.
>  I've just fixed up all the known bugs, so I should be posting v14
> later today, if all goes well.

I will definitely implement mem_access for PVH, however the platform I am 
targeting in the short term does not have EPT so I will have to go the shadow 
route.

Thanks,
Aravindh

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