Re: [Qemu-devel] [PATCH v2 10/16] postcopy: Load huge pages in one go

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Laurent Vivier (lvivier@redhat.com) wrote:
> On 06/02/2017 18:33, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > The existing postcopy RAM load loop already ensures that it
> > glues together whole host-pages from the target page size chunks sent
> > over the wire.  Modify the definition of host page that it uses
> > to be the RAM block page size and thus be huge pages where appropriate.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > Reviewed-by: Juan Quintela <quintela@redhat.com>
> > ---
> >  migration/ram.c | 13 ++++++++-----
> >  1 file changed, 8 insertions(+), 5 deletions(-)
> > 
> > diff --git a/migration/ram.c b/migration/ram.c
> > index ff448ef..88d9444 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -2342,7 +2342,7 @@ static int ram_load_postcopy(QEMUFile *f)
> >  {
> >      int flags = 0, ret = 0;
> >      bool place_needed = false;
> > -    bool matching_page_sizes = qemu_host_page_size == TARGET_PAGE_SIZE;
> > +    bool matching_page_sizes = false;
> 
> The false value is not obvious.
> Is gcc smart enough to detect you use "matching_page_sizes" (in the
> "switch ()") only when it has been really initialized (in the "if ()")?

4.8.5-8 on RHEL 7 doesn't like it if I drop the false.
(That took a bit of searching, RHEL 6 is OK, f25 is OK)
But generally I've found these ram-load loops are really good
for tickling gcc's paranoia.

> >      MigrationIncomingState *mis = migration_incoming_get_current();
> >      /* Temporary page that is later 'placed' */
> >      void *postcopy_host_page = postcopy_get_tmp_page(mis);
> > @@ -2372,8 +2372,11 @@ static int ram_load_postcopy(QEMUFile *f)
> >                  ret = -EINVAL;
> >                  break;
> >              }
> > +            matching_page_sizes = block->page_size == TARGET_PAGE_SIZE;
> >              /*
> > -             * Postcopy requires that we place whole host pages atomically.
> > +             * Postcopy requires that we place whole host pages atomically;
> > +             * these may be huge pages for RAMBlocks that are backed by
> > +             * hugetlbfs.
> >               * To make it atomic, the data is read into a temporary page
> >               * that's moved into place later.
> >               * The migration protocol uses,  possibly smaller, target-pages
> > @@ -2381,9 +2384,9 @@ static int ram_load_postcopy(QEMUFile *f)
> >               * of a host page in order.
> >               */
> >              page_buffer = postcopy_host_page +
> > -                          ((uintptr_t)host & ~qemu_host_page_mask);
> > +                          ((uintptr_t)host & (block->page_size - 1));
> >              /* If all TP are zero then we can optimise the place */
> > -            if (!((uintptr_t)host & ~qemu_host_page_mask)) {
> > +            if (!((uintptr_t)host & (block->page_size - 1))) {
> >                  all_zero = true;
> >              } else {
> >                  /* not the 1st TP within the HP */
> > @@ -2401,7 +2404,7 @@ static int ram_load_postcopy(QEMUFile *f)
> >               * page
> >               */
> >              place_needed = (((uintptr_t)host + TARGET_PAGE_SIZE) &
> > -                                     ~qemu_host_page_mask) == 0;
> > +                                     (block->page_size - 1)) == 0;
> >              place_source = postcopy_host_page;
> >          }
> >          last_host = host;
> > 
> 
> Reviewed-by: Laurent Vivier <lvivier@redhat.com>

Thanks.

Dave

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK