Re: Live migration leaves page tables read-only?

[John Byrne <john.l.byrne@hp.com>]

Ian Pratt wrote:
>  
>> Solaris implements PROT_NONE by entirely invalidating the PTE (ie. it
>> becomes zero). Hence our PTEs always had either zero or have the
> PRESENT
>> bit set.  The only exception to this was adding some fixage to allow
>> for the old Xen writable page table approach which temporarily made
>> the upper table non-PRESENT.
>>
>> So you can make not-present, but non-zero entries mean anything you
> want.
>> As long as it's the guest OS that creates the entries, we'll just not
> do
>> it.
> 
> Just to be confirm: in Solaris there are no not-present PTE's that
> contain machine addresses.
> 
> This means we need to implement the scheme that Keir suggested to enable
> the guest OS to tell xen/xc_save/restore about flags in not-present PTEs
> that should trigger a m2p conversion.
> 
> Ian 
> 

Ian,

Silly me. I thought "xc_linux_save" meant what it said. I haven't paid 
much attention to BSD or Solaris on Xen and didn't realize that went 
through the same path.

I'd really like to see this fixed for 3.0.4, at least for Linux, but I 
don't think I'm the person to implement a new "scheme" quickly to do it, 
but I'll try if someone wants to give me some advice on how to start.

On the subject of schemes, what about support for other architectures? 
Is there anything we should be thinking about for supporting guests with 
different page sizes, for instance?

John Byrne