On Fri, Jan 5, 2024 at 4:55 PM Jiri Denemark <[email protected]> wrote:

> During post-copy migration (once it actually switches to post-copy mode)
> dirty memory pages are continued to be migrated iteratively, while the
> destination can explicitly request a specific page to be migrated before
> the iterative process gets to it (which happens when a guest wants to
> read a page that was not migrated yet). Without this flag explicitly
> requested pages need to wait until all other pages already queued are
> transferred. Enabling this flag will instruct the hypervisor to create a
> separate migration channel for explicitly requested pages so that they
> can preempt the queue.
>
> Signed-off-by: Jiri Denemark <[email protected]>
> ---
>  include/libvirt/libvirt-domain.h | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/include/libvirt/libvirt-domain.h
> b/include/libvirt/libvirt-domain.h
> index 30cce85b29..ee23ae97a5 100644
> --- a/include/libvirt/libvirt-domain.h
> +++ b/include/libvirt/libvirt-domain.h
> @@ -1100,6 +1100,14 @@ typedef enum {
>       * Since: 8.5.0
>       */
>      VIR_MIGRATE_ZEROCOPY = (1 << 20),
> +
> +    /* Allow post-copy requests to preempt normal migration stream. This
> way
> +     * page faults on the destination hosts caused by accessing a memory
> page
> +     * that was not migrated yet should be handled faster.
>
> Based on this description and the commit message there doesn't seem to be
any reason why the user should not enable that. And in that case I think
the users will be better off when libvirt auto-enables this.
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