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

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

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.
Since marking a page broken will also mark it dirty, I suppose what we 
could do is, on the last iteration, clear the dirty bitmap after getting 
the list of pages but before copying them; and then check the bit in the 
bitmap corresponding to the pfn after copying it.
But on the whole, is that really that much *faster* to do it that way?  
Either way you're still adding a single hypercall to the whole thing, 
and one iteration of a loop for each page; but in Jinsong's patch, the 
loop is done all at once, so presumably the compiler / processor will be 
able to do make better use of loop unrolling / registers / the pipeline 
/ branch prediction / caches &c.
The only way to avoid the loop would be to add an extra "have any pages 
been marked broken / dirty" bit somewhere, which is probably more 
trouble than it's worth.
 -George

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