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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2012-11-19 at 15:29 +0000, George Dunlap wrote:
> On 19/11/12 09:55, Ian Campbell wrote:
> > If we get to this stage then haven't we either already sent something
> > over the wire for this page or marked it as dirty when we tried and
> > failed to send it?
> >
> > In the former case we don't care that the page is now broken on the
> > source since the target has got a good pre-breakage copy.
> >
> > In the latter case could we not set a flag at the same time as we mark
> > the page dirty which means "go round at least one more time"?
> 
> Yeah -- on the last iteration, the VM itself has to be paused; if any 
> pages get broken after that, it doesn't really matter, does it? The real 
> thing is to have a consistent "snapshot" of behavior.
> 
> I guess the one potentially tricky case to worry about is whether to 
> deliver an MCE to the guest on restore.  Consider the following scenario:
> 
> - Page A is modified (and marked dirty)
> - VM paused for last iteration
> - Page breaks, is marked broken in the p2m
> - Save code sends page A
> 
> In that case, the save code would send a "broken" page, and the restore 
> code would mark a page as broken, and we *would* want to deliver an MCE 
> on the far side.  But suppose the last two steps were reversed:
> 
> - Page A modified
> - VM paused for last iteration
> - Save code sends page A
> - Page breaks, marked broken in the p2m
> 
> In that case, when the save code sends page A, it will send a good page; 
> there's no need to mark it broken, or to send the guest an MCE.

I guess you'd want to err on the side of stopping using a good page, as
opposed to continuing to use a bad page? i.e. its better to take a
spurious vMCE than to not take an actual one.

I'm not actually sure what a guest does with a vMCE, I guess it does
some sort of memory exchange to give the bad page back to the h/v and
get a good page in return? If the hypervisor thinks the old page is ok
rather than bad I guess it'll just put it in the free list instead of
the bad list?

> 
> Am I understanding the situation correctly, Jinsong?
> 
>   -George



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