Re: [Qemu-devel] [PATCH v8 42/54] Postcopy: Use helpers to map pages during migration

[Dr. David Alan Gilbert]

* Juan Quintela (address@hidden) wrote:
> "Dr. David Alan Gilbert (git)" <address@hidden> wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> >
> > In postcopy, the destination guest is running at the same time
> > as it's receiving pages; as we receive new pages we must put
> > them into the guests address space atomically to avoid a running
> > CPU accessing a partially written page.
> >
> > Use the helpers in postcopy-ram.c to map these pages.
> >
> > qemu_get_buffer_in_place is used to avoid a copy out of qemu_file
> > in the case that postcopy is going to do a copy anyway.
> >
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > ---
> >  migration/ram.c | 128 
> > +++++++++++++++++++++++++++++++++++++++++++++-----------
> >  1 file changed, 103 insertions(+), 25 deletions(-)
> >

> > diff --git a/migration/ram.c b/migration/ram.c
> > +        postcopy_place_needed = false;
> > +        if (flags & (RAM_SAVE_FLAG_COMPRESS | RAM_SAVE_FLAG_PAGE |
> > +                     RAM_SAVE_FLAG_COMPRESS_PAGE | RAM_SAVE_FLAG_XBZRLE)) {
> > +            host = host_from_stream_offset(f, mis, addr, flags);
> > +            if (!host) {
> > +                error_report("Illegal RAM offset " RAM_ADDR_FMT, addr);
> > +                ret = -EINVAL;
> > +                break;
> > +            }
> > +            page_buffer = host;
> 
> You can move this bit of code here in a different patch, makes review easier.
> all_zero can also be on that patch.

Done; this is now 'ram_load: Factor out host_from_stream_offset call and check'

> 
> You are reusingh ram_load, but have lots and lots of
> 
> if (postcopy_running) {
> 
> } else {
> 
> }
> 
> I think that it would be easier to just have:
> 
> if (postcopy_running) {
>      ram_load_postcopy()
> } else {
>      ram_load_precopy{}
> }
> 
> You duplicate a bit of code, but remove lots of ifs from the equation,
> not sure which one is really easier.  I just hate bits like the
> following one.
> 
> > @@ -2062,32 +2123,36 @@ static int ram_load(QEMUFile *f, void *opaque, int 
> > version_id)
> >              }
> >              break;
> >          case RAM_SAVE_FLAG_COMPRESS:
> >              ch = qemu_get_byte(f);
> > -            ram_handle_compressed(host, ch, TARGET_PAGE_SIZE);
> > +            if (!postcopy_running) {
> > +                ram_handle_compressed(host, ch, TARGET_PAGE_SIZE);
> > +            } else {
> > +                memset(page_buffer, ch, TARGET_PAGE_SIZE);
> > +                if (ch) {
> > +                    all_zero = false;
> > +                }
> > +            }
> 


Yeh, I've split that out now into ram_load_postcopy (called from just
before the main loop in ram_load); as you say it is a bit bigger,
but clearer.

> > +            if (postcopy_running) {
> 
> 
> As discussed on irc, I still think that having a RAM_SAVE_HOST_PAGE make
> everything much, much clearer and easier, but I agree that is not
> trivial with current code.

(I've moved this comment down a bit in this reply).
Actually, now that the postcopy load code is in a separate routine, it might
be possible to reorder things a bit since we know all of these pages are 
host-page-sized.

Dave
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK