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

[George Dunlap <dunlapg@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.

 -George

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