Re: [Qemu-devel] [PATCH v1] migration/ram.c: do not set 'postcopy_running' in POSTCOPY_INCOMING_END

[Peter Xu <peterx@redhat.com>]

On Fri, Nov 10, 2017 at 06:35:16PM -0200, Daniel Henrique Barboza wrote:
> When migrating a VM with 'migrate_set_capability postcopy-ram on'
> a postcopy_state is set during the process, ending up with the
> state POSTCOPY_INCOMING_END when the migration is over. This
> postcopy_state is taken into account inside ram_load to check
> how it will load the memory pages. This same ram_load is called when
> in a loadvm command.
> 
> Inside ram_load, the logic to see if we're at postcopy_running state
> is:
> 
> postcopy_running = postcopy_state_get() >= POSTCOPY_INCOMING_LISTENING
> 
> postcopy_state_get() returns this enum type:
> 
> typedef enum {
>     POSTCOPY_INCOMING_NONE = 0,
>     POSTCOPY_INCOMING_ADVISE,
>     POSTCOPY_INCOMING_DISCARD,
>     POSTCOPY_INCOMING_LISTENING,
>     POSTCOPY_INCOMING_RUNNING,
>     POSTCOPY_INCOMING_END
> } PostcopyState;
> 
> In the case where ram_load is executed and postcopy_state is
> POSTCOPY_INCOMING_END, postcopy_running will be set to 'true' and
> ram_load will behave like a postcopy is in progress. This scenario isn't
> achievable in a migration but it is reproducible when executing
> savevm/loadvm after migrating with 'postcopy-ram on', causing loadvm
> to fail with Error -22:
> 
> Source:
> 
> (qemu) migrate_set_capability postcopy-ram on
> (qemu) migrate tcp:127.0.0.1:4444
> 
> Dest:
> 
> (qemu) migrate_set_capability postcopy-ram on
> (qemu)
> ubuntu1704-intel login:
> Ubuntu 17.04 ubuntu1704-intel ttyS0
> 
> ubuntu1704-intel login: (qemu)
> (qemu) savevm test1
> (qemu) loadvm test1
> Unknown combination of migration flags: 0x4 (postcopy mode)
> error while loading state for instance 0x0 of device 'ram'
> Error -22 while loading VM state
> (qemu)
> 
> This patch fixes this problem by changing a bit the semantics
> of postcopy_running inside ram_load, verifying first if
> we're not in the POSTCOPY_INCOMING_END state. In this case,
> postcopy_running is set to 'false'.
> 
> Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
> ---
>  migration/ram.c | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 8620aa400a..43ed719668 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -2803,13 +2803,21 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
>      int flags = 0, ret = 0, invalid_flags = 0;
>      static uint64_t seq_iter;
>      int len = 0;
> -    /*
> -     * If system is running in postcopy mode, page inserts to host memory must
> -     * be atomic
> -     */
> -    bool postcopy_running = postcopy_state_get() >= POSTCOPY_INCOMING_LISTENING;
> -    /* ADVISE is earlier, it shows the source has the postcopy capability on */
> -    bool postcopy_advised = postcopy_state_get() >= POSTCOPY_INCOMING_ADVISE;
> +    bool postcopy_advised = false, postcopy_running = false;
> +    uint8_t postcopy_state = postcopy_state_get();
> +
> +    if (postcopy_state != POSTCOPY_INCOMING_END) {
> +        /*
> +         * If system is running in postcopy mode, page inserts to host memory
> +         * must be atomic
> +         */
> +        postcopy_running = postcopy_state >= POSTCOPY_INCOMING_LISTENING;
> +
> +        /* ADVISE is earlier, it shows the source has the postcopy
> +         * capability on
> +         */
> +        postcopy_advised = postcopy_state >= POSTCOPY_INCOMING_ADVISE;
> +    }

For me, this is not that clear.  I would prefer something simpler like:

  PostcopyState state = postcopy_state_get();
  bool postcopy_running = state >= POSTCOPY_INCOMING_LISTENING && \
                          state < POSTCOPY_INCOMING_END;

Or we can introduce postcopy_is_running() helper.  Same thing to
postcopy_advised.  Thanks,

>  
>      seq_iter++;
>  
> -- 
> 2.13.6
> 
> 

-- 
Peter Xu