Re: [Qemu-devel] [PATCH v3 17/29] vhost+postcopy: Send requests to source for shared pages

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

* Peter Xu (peterx@redhat.com) wrote:
> On Fri, Feb 16, 2018 at 01:16:13PM +0000, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Send requests back to the source for shared page requests.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  migration/migration.h    |  2 ++
> >  migration/postcopy-ram.c | 31 ++++++++++++++++++++++++++++---
> >  migration/postcopy-ram.h |  3 +++
> >  migration/trace-events   |  2 ++
> >  4 files changed, 35 insertions(+), 3 deletions(-)
> > 
> > diff --git a/migration/migration.h b/migration/migration.h
> > index d158e62cf2..457bf37ec2 100644
> > --- a/migration/migration.h
> > +++ b/migration/migration.h
> > @@ -46,6 +46,8 @@ struct MigrationIncomingState {
> >      int       userfault_quit_fd;
> >      QEMUFile *to_src_file;
> >      QemuMutex rp_mutex;    /* We send replies from multiple threads */
> > +    /* RAMBlock of last request sent to source */
> > +    RAMBlock *last_rb;
> >      void     *postcopy_tmp_page;
> >      void     *postcopy_tmp_zero_page;
> >      /* PostCopyFD's for external userfaultfds & handlers of shared memory */
> > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > index d118b78bf5..277ff749a0 100644
> > --- a/migration/postcopy-ram.c
> > +++ b/migration/postcopy-ram.c
> > @@ -534,6 +534,31 @@ static int ram_block_enable_notify(const char *block_name, void *host_addr,
> >      return 0;
> >  }
> >  
> > +/*
> > + * Callback from shared fault handlers to ask for a page,
> > + * the page must be specified by a RAMBlock and an offset in that rb
> > + */
> > +int postcopy_request_shared_page(struct PostCopyFD *pcfd, RAMBlock *rb,
> > +                                 uint64_t client_addr, uint64_t rb_offset)
> > +{
> > +    size_t pagesize = qemu_ram_pagesize(rb);
> > +    uint64_t aligned_rbo = rb_offset & ~(pagesize - 1);
> > +    MigrationIncomingState *mis = migration_incoming_get_current();
> > +
> > +    trace_postcopy_request_shared_page(pcfd->idstr, qemu_ram_get_idstr(rb),
> > +                                       rb_offset);
> > +    /* TODO: Check bitmap to see if we already have the page */
> > +    if (rb != mis->last_rb) {
> > +        mis->last_rb = rb;
> > +        migrate_send_rp_req_pages(mis, qemu_ram_get_idstr(rb),
> > +                                  aligned_rbo, pagesize);
> > +    } else {
> > +        /* Save some space */
> > +        migrate_send_rp_req_pages(mis, NULL, aligned_rbo, pagesize);
> > +    }
> > +    return 0;
> > +}
> > +
> 
> So IIUC this can only be called within the page fault thread or there
> can be race.  Is there a way to guarantee this?  Or do we need a
> comment for that?

I don't think there's a way to guarantee it - especially since it has
to be called by the device-specific shared handlers in another file -

I've updated the comment to:

/*
 * Callback from shared fault handlers to ask for a page,
 * the page must be specified by a RAMBlock and an offset in that rb
 * Note: Only for use by shared fault handlers (in fault thread)
 */

Dave

> >  /*
> >   * Handle faults detected by the USERFAULT markings
> >   */
> > @@ -544,9 +569,9 @@ static void *postcopy_ram_fault_thread(void *opaque)
> >      int ret;
> >      size_t index;
> >      RAMBlock *rb = NULL;
> > -    RAMBlock *last_rb = NULL; /* last RAMBlock we sent part of */
> >  
> >      trace_postcopy_ram_fault_thread_entry();
> > +    mis->last_rb = NULL; /* last RAMBlock we sent part of */
> >      qemu_sem_post(&mis->fault_thread_sem);
> >  
> >      struct pollfd *pfd;
> > @@ -634,8 +659,8 @@ static void *postcopy_ram_fault_thread(void *opaque)
> >               * Send the request to the source - we want to request one
> >               * of our host page sizes (which is >= TPS)
> >               */
> > -            if (rb != last_rb) {
> > -                last_rb = rb;
> > +            if (rb != mis->last_rb) {
> > +                mis->last_rb = rb;
> >                  migrate_send_rp_req_pages(mis, qemu_ram_get_idstr(rb),
> >                                           rb_offset, qemu_ram_pagesize(rb));
> >              } else {
> > diff --git a/migration/postcopy-ram.h b/migration/postcopy-ram.h
> > index dbc2ee1f2b..4c63f20df4 100644
> > --- a/migration/postcopy-ram.h
> > +++ b/migration/postcopy-ram.h
> > @@ -162,5 +162,8 @@ struct PostCopyFD {
> >   */
> >  void postcopy_register_shared_ufd(struct PostCopyFD *pcfd);
> >  void postcopy_unregister_shared_ufd(struct PostCopyFD *pcfd);
> > +/* Callback from shared fault handlers to ask for a page */
> > +int postcopy_request_shared_page(struct PostCopyFD *pcfd, RAMBlock *rb,
> > +                                 uint64_t client_addr, uint64_t offset);
> >  
> >  #endif
> > diff --git a/migration/trace-events b/migration/trace-events
> > index 1e617ad7a6..7c910b5479 100644
> > --- a/migration/trace-events
> > +++ b/migration/trace-events
> > @@ -198,6 +198,8 @@ postcopy_ram_incoming_cleanup_closeuf(void) ""
> >  postcopy_ram_incoming_cleanup_entry(void) ""
> >  postcopy_ram_incoming_cleanup_exit(void) ""
> >  postcopy_ram_incoming_cleanup_join(void) ""
> > +postcopy_request_shared_page(const char *sharer, const char *rb, uint64_t rb_offset) "for %s in %s offset 0x%"PRIx64
> > +
> >  save_xbzrle_page_skipping(void) ""
> >  save_xbzrle_page_overflow(void) ""
> >  ram_save_iterate_big_wait(uint64_t milliconds, int iterations) "big wait: %" PRIu64 " milliseconds, %d iterations"
> > -- 
> > 2.14.3
> > 
> 
> -- 
> Peter Xu
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK