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

[George Dunlap <george.dunlap@eu.citrix.com>]

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.

Am I understanding the situation correctly, Jinsong?

  -George