Re: Live migration leaves page tables read-only?

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

Ian,

I haven't noticed a fix. Is someone working on this bug or should I open 
a bugzilla for it, so it isn't forgotten?

John Byrne

Ian Pratt wrote:
>  
>> I finally ran down the problem. SAP is protecting the pages PROT_NONE,
>> so the page-present bit in the pte is not set and
>> canonicalize/uncanonicalize code in save/restore ignore the pte. I've
>> attached a patch. It is possible that this change should be made to
> the
>> l1e tests in xc_ptrace.c; I'm not sure.
> 
> That's a good catch, thanks. Interesting that we hadn't seen this
> before.
> 
> Although your patch works today, it will break when we add PSE (super
> page) support for PV guests as it will confuse PROT_NONE with PSE.
> Assuming PROT_NONE only makes sense for L1 entries, we can probably gate
> the tests on whether the page table page is an L1 or not to fix this.
> 
> However, it does point out an issue for other OSes: Taking this patch
> effectively makes Linux's PROT_NONE (flags 0x80 for a not present PTE)
> part of the Xen API. We need to find out whether this is compatible with
> *BSD and Solaris' use of flags for not present ptes.
> 
> Ian
>