Re: [Qemu-devel] [PATCH 1/4] migration: add RAMBlock's offset validation

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 10 Jan 2019 20:14:19 +0000
"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> * Yury Kotov (yury-kotov@yandex-team.ru) wrote:
> > RAM migration has a RAMBlock validation stage (flag RAM_SAVE_FLAG_MEM_SIZE).
> > In this stage QEMU checks further information about RAMBlock:
> > 1. Presence (by idstr),
> > 2. Length (trying to resize, when differs),
> > 3. Optional page size.
> > 
> > This patch adds a check for RAMBlock's offset. Currently we check it during
> > RAM pages loading - every RAM page has an offset in its header. But there is a
> > case when we don't send RAM pages (see below).
> > 
> > The following commits introduce a capability (ignore-external) to skip some
> > RAM blocks from migration. In such case the migration stream contains only
> > meta information about RAM blocks to validate them. So, the only way to check
> > block's offset is to send it explicitly.
> > 
> > Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>  
> 
> But why check that offsets match?  THey aren't supposed to!
> Offset's are entirely private to each qemu and they're allowed to be
> different; the only requirement is that the length and name of each
> RAMBlock matches, then all the operations we do over the migration
> stream are relative to the start of the block.
> 
> One example where they are validly different is where you hotplug some
> RAM, so for example:
> 
> 
>   source qemu
>       -M 4G
>       hotplug PCI card
>       hotplug 2G
> 
>   destination qemu
>       -M 4G
>       PCI card declared on the command line
>       extra 2G declared on the command line
> 
> The offsets are different but we can migrate that case fine.
PCI mappings are updated after migration is done, to make DST match SRC
(get_pci_config_device) that mapping is irrelevant here, but I was under
impression that offset in terms of MemoryRegion (GPA) was passed in
migration stream, even if we do not update memory mappings for RAM
(mapping depends on hotplug/CLI order or explicit addr on CLI for memory
devices).

> 
> Dave
> 
> > ---
> >  migration/ram.c | 15 +++++++++++++--
> >  1 file changed, 13 insertions(+), 2 deletions(-)
> > 
> > diff --git a/migration/ram.c b/migration/ram.c
> > index 7e7deec4d8..39629254e1 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -3171,6 +3171,7 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
> >          if (migrate_postcopy_ram() && block->page_size != qemu_host_page_size) {
> >              qemu_put_be64(f, block->page_size);
> >          }
> > +        qemu_put_be64(f, block->offset);
> >      }
> >  
> >      rcu_read_unlock();
> > @@ -4031,7 +4032,7 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
> >  
> >      seq_iter++;
> >  
> > -    if (version_id != 4) {
> > +    if (version_id < 4) {
> >          ret = -EINVAL;
> >      }
> >  
> > @@ -4132,6 +4133,16 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
> >                              ret = -EINVAL;
> >                          }
> >                      }
> > +                    if (version_id >= 5) {
> > +                        ram_addr_t offset;
> > +                        offset = qemu_get_be64(f);
> > +                        if (block->offset != offset) {
> > +                            error_report("Mismatched RAM block offset %s "
> > +                                         "%" PRId64 "!= %" PRId64,
> > +                                         id, offset, (uint64_t)block->offset);
> > +                            ret = -EINVAL;
> > +                        }
> > +                    }
> >                      ram_control_load_hook(f, RAM_CONTROL_BLOCK_REG,
> >                                            block->idstr);
> >                  } else {
> > @@ -4363,5 +4374,5 @@ static SaveVMHandlers savevm_ram_handlers = {
> >  void ram_mig_init(void)
> >  {
> >      qemu_mutex_init(&XBZRLE.lock);
> > -    register_savevm_live(NULL, "ram", 0, 4, &savevm_ram_handlers, &ram_state);
> > +    register_savevm_live(NULL, "ram", 0, 5, &savevm_ram_handlers, &ram_state);
> >  }
> > -- 
> > 2.20.1
> >   
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK