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



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

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