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Re: [PATCH] tools/libxenguest: Fix migration's debug option

  • To: Olaf Hering <olaf@xxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Mon, 5 Jul 2021 10:23:00 +0200
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  • Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Mon, 05 Jul 2021 08:23:07 +0000
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On 05.07.2021 10:02, Olaf Hering wrote:
> Am Mon, 5 Jul 2021 09:57:21 +0200
> schrieb Jan Beulich <jbeulich@xxxxxxxx>:
>> What is "the grant problem" referring to here? Neither anything above
>> nor the offending original commit has any reference to grants, or a
>> problem with them.
> When the guest is paused during final transit, the backends will
> continue to write into domU memory. As a result the final additional
> iteration to verify memory on both sides will always see errors.

I see. A similar problem then exists with at least the FIFO event
channel per-vCPU control blocks?

> The code has no way to know for which pfn such mismatches in page
> content can safely be ignored.

Well, in principle this can be known, but it's expensive: For a
paused domain the grant table can't change anymore. Any pages
referenced by a valid non-r/o grant table entry could in principle




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