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

["Liu, Jinsong" <jinsong.liu@xxxxxxxxx>]

Well, let me elaborate more my thought about broken page occur at last iter:

Theoretically we can separate 'broken page at last iter' into several 
sub-cases, and optimize case by case:
1. if the page (which will break at last iter) is not on dirty bitmap (of last 
iter) --> do nothing is OK, target will happily run w/o broken page;
2. if the page (which will break at last iter) is on dirty bitmap (of last iter)
  2.1 if at last iter, vmce occur after page copy --> do nothing is OK, target 
happily run w/o broken page;
  2.2 if vmce occur beofre pfn_type transfer --> do nothing is OK, V2 patch has 
correctly handle the case, target will set p2m broken correctly;
  2.3 if vmce occur after pfn_type transfer and before copy page to target --> 
we need handle this case;

Practically considering it's rare enough, and code complication, we handle it 
in a simple way (not so optimized but enough for real life):
- we don't distinguish if the page is in dirty bitmap of last iter;
- we don't prefer adding new hypercall for this corner case, instead we'd like 
to re-use currently existed hypercall;
- if vmce occur at last iter, we do 1~2 more iter.
This is what V3 patch does now.

Thoughts?

Thanks,
Jinsong
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