On Fri, Jan 05, 2024 at 17:31:50 +0100, Jiri Denemark wrote:
> On Fri, Jan 05, 2024 at 17:11:57 +0100, Peter Krempa wrote:
> > On Fri, Jan 5, 2024 at 4:55 PM Jiri Denemark <jdenemar@xxxxxxxxxx> 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 <jdenemar@xxxxxxxxxx>
> > > ---
> > > 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.
>
> Well yes, ideally you want this enabled all the time. But Peter Xu did
> not sound like it was ready for being always enabled. I guess I'll check
> with him if it's still the case or not.
So Peter confirmed we may enable postcopy-preempt for all post-copy
migrations (as long as the capability is supported). Reworked patch
series is coming in a few moments.
Jirka
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