Re: [PATCH v7 05/12] migration: Make ram_save_target_page() a pointer

[Juan Quintela]

Leonardo Brás <leobras@redhat.com> wrote:
> On Tue, 2022-08-02 at 08:39 +0200, Juan Quintela wrote:
>> We are going to create a new function for multifd latest in the series.
>> 
>> Signed-off-by: Juan Quintela <quintela@redhat.com>
>> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>> Signed-off-by: Juan Quintela <quintela@redhat.com>
>
> Double Signed-off-by again.
>
>> ---
>>  migration/ram.c | 13 +++++++++----
>>  1 file changed, 9 insertions(+), 4 deletions(-)
>> 
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 85d89d61ac..499d9b2a90 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -310,6 +310,9 @@ typedef struct {
>>      bool preempted;
>>  } PostcopyPreemptState;
>>  
>> +typedef struct RAMState RAMState;
>> +typedef struct PageSearchStatus PageSearchStatus;
>> +
>>  /* State of RAM for migration */
>>  struct RAMState {
>>      /* QEMUFile used for this migration */
>> @@ -372,8 +375,9 @@ struct RAMState {
>>       * is enabled.
>>       */
>>      unsigned int postcopy_channel;
>> +
>> +    int (*ram_save_target_page)(RAMState *rs, PageSearchStatus *pss);
>>  };
>> -typedef struct RAMState RAMState;
>>  
>>  static RAMState *ram_state;
>>  
>> @@ -2255,14 +2259,14 @@ static bool save_compress_page(RAMState *rs, 
>> RAMBlock *block, ram_addr_t offset)
>>  }
>>  
>>  /**
>> - * ram_save_target_page: save one target page
>> + * ram_save_target_page_legacy: save one target page
>>   *
>>   * Returns the number of pages written
>>   *
>>   * @rs: current RAM state
>>   * @pss: data about the page we want to send
>>   */
>> -static int ram_save_target_page(RAMState *rs, PageSearchStatus *pss)
>> +static int ram_save_target_page_legacy(RAMState *rs, PageSearchStatus *pss)
>>  {
>>      RAMBlock *block = pss->block;
>>      ram_addr_t offset = ((ram_addr_t)pss->page) << TARGET_PAGE_BITS;
>> @@ -2469,7 +2473,7 @@ static int ram_save_host_page(RAMState *rs, 
>> PageSearchStatus *pss)
>>  
>>          /* Check the pages is dirty and if it is send it */
>>          if (migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
>> -            tmppages = ram_save_target_page(rs, pss);
>> +            tmppages = rs->ram_save_target_page(rs, pss);
>>              if (tmppages < 0) {
>>                  return tmppages;
>>              }
>> @@ -3223,6 +3227,7 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>>      ram_control_before_iterate(f, RAM_CONTROL_SETUP);
>>      ram_control_after_iterate(f, RAM_CONTROL_SETUP);
>>  
>> +    (*rsp)->ram_save_target_page = ram_save_target_page_legacy;
>>      ret =  multifd_send_sync_main(f);
>>      if (ret < 0) {
>>          return ret;
>
>
> So, IIUC:
> - Rename ram_save_target_page -> ram_save_target_page_legacy
> - Add a function pointer to RAMState (or a callback)
> - Assign function pointer = ram_save_target_page_legacy at setup
> - Replace ram_save_target_page() by indirect function call using above 
> pointer.
>
> I could see no issue in this, so I belive it works fine.
>
> The only thing that concerns me is the name RAMState.

Every device state is setup in RAMState.

> IMHO, a struct named RAMState is supposed to just reflect the state of ram (or
> according to this struct's comments, the state of RAM for migration. Having a
> function pointer here that saves a page seems counterintuitive, since it does
> not reflect the state of RAM.

The big problem for adding another struct is that we would have to
change all the callers, or yet another global variable.  Both are bad
idea in my humble opinion.

> Maybe we could rename the struct, or even better, create another struct that
> could look something like this:
>
> struct RAMMigration {
>     RAMState state;
>     int (*ram_save_target_page)(RAMState *rs, PageSearchStatus *pss);
>     /* Other callbacks or further info.*/
> }
>
> What do you think about it?

Really this depends on configuration.  What is setup for qemu
migration.  I think this is the easiest way to do it, we can add a new
struct, but it gets everything much more complicated:

- the value that we receive in ram_save_setup() is a RAMState
- We would have to change all the callers form
  * ram_save_iterate()
  * ram_find_and_save_block()
  * ram_save_host_page()

So I think it is quite a bit of churn for not a lot of gain.

Later, Juan.