[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH V3] X86/vMCE: handle broken page with regard to migration
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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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