Re: [Xen-devel] [PATCH V3] X86/vMCE: handle broken page with regard to migration

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2012-11-21 at 11:34 +0000, George Dunlap wrote:
> On 20/11/12 18:42, Ian Jackson wrote:
> > Liu, Jinsong writes ("RE: [Xen-devel] [PATCH V3] X86/vMCE: handle broken 
> > page with regard to migration"):
> >> Ian Jackson wrote:
> >>> Liu, Jinsong writes ("RE: [Xen-devel] [PATCH V3] X86/vMCE: handle
> >>> broken page with regard to migration"):
> >>>> No, at last lter, there are 4 points:
> >>>> 1. start last iter
> >>>> 2. get and transfer pfn_type to target
> >>>> 3. copy page to target
> >>>> 4. end last iter
> > ...
> >> It indeed checks mce after point 3 for each page, but what's the
> >> advantage of keeping a separate list?
> > It avoids yet another loop over all the pages.  Unless I have
> > misunderstood.  Which I may have, because: if it checks for mce after
> > point 3 then surely that is sufficient ?  We don't need to worry about
> > mces after that check.
> 
> It's sufficient, but wouldn't each check require a separate hypercall?  
> That would surely be slower than just a single hypercall and a loop 
> (which is what Jinsong's patch does).
> 
> We don't actually need a list -- I think we just need to know, "Have any 
> pages broken between reading the p2m table ( xc_get_pfn_type_batch() ); 
> if so, we do another full iteration.

If a page fails between 2. and 3. above then what happens at point 3? I
presume we can't map and send the page (since it is broken), do we get
some sort of failure to map?

What happens if the failure occurs during stage 3, i.e. while the page
is mapped and we are reading from it?

Ian.


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